Specific proteome changes in platelets from individuals with GATA1-, GFI1B-, and RUNX1-linked bleeding disorders.

Mutations in the transcription factors GATA binding factor 1 (GATA1), growth factor independence 1B (GFI1B), and Runt-related transcription factor 1 (RUNX1) cause familial platelet and bleeding disorders. Mutant platelets exhibit common abnormalities including an α-granule reduction resulting in a grayish appearance in blood smears. This suggests that similar pathways are deregulated by different transcription factor mutations. To identify common factors, full platelet proteomes from 11 individuals with mutant GATA1R216Q, GFI1BQ287*, RUNX1Q154Rfs, or RUNX1TD2-6 and 28 healthy controls were examined by label-free quantitative mass spectrometry. In total, 2875 platelet proteins were reliably quantified. Clustering analysis of more than 300 differentially expressed proteins revealed profound differences between cases and controls. Among cases, 44 of 143 significantly downregulated proteins were assigned to platelet function, hemostasis, and granule biology, in line with platelet dysfunction and bleedings. Remarkably, none of these proteins were significantly diminished in all affected cases. Similarly, no proteins were commonly overrepresented in all affected cases compared with controls. These data indicate that the studied transcription factor mutations alter platelet proteomes in distinct largely nonoverlapping manners. This work provides the quantitative landscape of proteins that affect platelet function when deregulated by mutated transcription factors in inherited bleeding disorders.

Hemolysis in the spleen drives erythrocyte turnover.

Red pulp macrophages (RPMs) of the spleen mediate turnover of billions of senescent erythrocytes per day. However, the molecular mechanisms involved in sequestration of senescent erythrocytes, their recognition, and their subsequent degradation by RPMs remain unclear. In this study, we provide evidence that the splenic environment is of substantial importance in facilitating erythrocyte turnover through induction of hemolysis. Upon isolating human spleen RPMs, we noted a substantial lack of macrophages that were in the process of phagocytosing intact erythrocytes. Detailed characterization of erythrocyte and macrophage subpopulations from human spleen tissue led to the identification of erythrocytes that are devoid of hemoglobin, so-called erythrocyte ghosts. By using in vivo imaging and transfusion experiments, we further confirmed that senescent erythrocytes that are retained in the spleen are subject to hemolysis. In addition, we showed that erythrocyte adhesion molecules, which are specifically activated on aged erythrocytes, cause senescent erythrocytes to interact with extracellular matrix proteins that are exposed within the splenic architecture. Such adhesion molecule-driven retention of senescent erythrocytes under low shear conditions was found to result in steady shrinkage of the cell and ultimately resulted in hemolysis. In contrast to intact senescent erythrocytes, the remnant erythrocyte ghost shells were prone to recognition and breakdown by RPMs. These data identify hemolysis as a key event in the turnover of senescent erythrocytes, which alters our current understanding of how erythrocyte degradation is regulated.