Simultaneous Loss of Mouse Dematin and beta-Adducin Results in Severe Erythrocyte Fragility and Hemolytic Anemia.

Abstract

The mechanical strength and stability of the erythrocyte membrane are regulated by a network of proteins that participate in both horizontal and vertical interactions. The actin-containing junctional complexes, located at the tail ends of spectrin molecules, serve as the critical regulatory nodes for the maintenance of membrane stability. Dematin and beta-adducin, two actin-binding proteins of the junctional complex, are known to play essential roles in the regulation of erythrocyte shape and membrane stability, as revealed recently by the development of mouse knockout models. Here, we show that simultaneous loss of functional dematin (headpiece domain deletion) and beta-adducin results in severe fragility and abnormal shape of erythrocytes, despite the presence of major skeletal proteins. Adducin/Dematin Double Knockout (ADKO) mice are viable and can be distinguished at birth by their pallor with pronounced spleomegaly and regenerative hematopoiesis. Hematological evaluations show a reduction of erythrocytes, reduced hematocrit and hemoglobin, and a ~52% increase in the number of reticulocytes. The presence of a variety of misshapen and fragmented erythrocytes in the ADKO mice correlates with increased osmotic fragility and reduced erythrocyte life span in vivo. Despite an apparently normal composition of ghosts and skeletal proteins, the retention of spectrin in the ADKO erythrocyte plasma membrane is significantly compromised. Atomic force microscopy (AFM) revealed similar volume parameters in the four genotypes examined, but an increased grain size, and a decreased filament number in the ADKO erythrocyte membrane. In addition, highly aggregated, disassembled, and irregular features were visualized by AFM in the ADKO erythrocyte membrane. Staining of filamentous actin provided further evidence for the existence of large protein aggregates in the ADKO erythrocyte membrane. Together, these results demonstrate a crucial function of dematin and beta-adducin in the maintenance of erythrocyte shape and membrane stability, and more importantly, suggest the existence of an alternate mechanism for the linkage of junctional complexes to the plasma membrane.