GRASP55 Is Dispensable for Normal Hematopoiesis but Necessary for Myc-Dependent Leukemic Growth.

Grasp55 is a ubiquitous Golgi stacking protein involved in autophagy, protein trafficking, and glucose deprivation sensing. The function of Grasp55 in protein trafficking has been attributed to its PDZ-mediated interaction with the C-terminal PDZ-binding motifs of protein cargos. We have recently shown that such an interaction occurs between Grasp55 and the adhesion molecule Jam-C, which plays a central role in stemness maintenance of hematopoietic and spermatogenic cells. Accordingly, we have found that Grasp55-deficient mice suffer from spermatogenesis defects similar to Jam-C knockout mice. However, whether Grasp55 is involved in the maintenance of immunohematopoietic homeostasis through regulation of protein transport and Jam-C expression remains unknown. In this study, we show that Grasp55 deficiency does not affect hematopoietic stem cell differentiation, engraftment, or mobilization, which are known to depend on expression of Grasp55-dependent protein cargos. In contrast, using an Myc-dependent leukemic model addicted to autophagy, we show that knockdown of Grasp55 in leukemic cells reduces spleen and bone marrow tumor burden upon i.v. leukemic engraftment. This is not due to reduced homing of Grasp55-deficient cells to these organs but to increased spontaneous apoptosis of Grasp55-deficient leukemic cells correlated with increased sensitivity of the cells to glucose deprivation. These results show that Grasp55 plays a role in Myc-transformed hematopoietic cells but not in normal hematopoietic cells in vivo.