RASA3 Regulates Stage-Specific AKT Signaling and Cell Cycle Progression in Mammalian Erythropoiesis
Authors: Brindley Elena C. , Hartman Emily , Papoin Julien , Lipton Jeffrey Michael , Peters Luanne L. , Blanc Lionel 
Published: November 2020

Inherited bone marrow failure syndromes (IBMFS) are a heterogenous group of disorders characterized by dysregulated hematopoiesis across various lineages, predisposition to malignancy, and diverse syndromic features. The scat (severe combined anemia and thrombocytopenia) mouse model has been characterized as a unique model of IBMFS. Scat carries an autosomal recessive missense mutation in the Rasa3 gene that results in RASA3 mislocalization and loss of function. RASA3 functions as a Ras-GTPase activating protein, and its loss of function in scat results in increased erythroid Ras activity, increased reactive oxygen species, and altered cell cycle progression, all culminating in delayed terminal erythroid differentiation. However, the precise mechanism of RASA3 regulation of erythroid differentiation remains undefined, and elucidation of this mechanism is crucial to identifying new therapeutic targets in inherited anemia.
Considering the role of RASA3 as regulator of Ras signaling and cell cycle progression, and the importance of these processes to erythroid differentiation, we sought to first characterize the coordination of Ras signaling pathways and cell cycle progression in normal murine erythropoiesis, then observe how loss of RASA3 function alters this regulatory axis. We observed that wild type (WT) erythroblasts demonstrate population-specific influence of ERK and PI3K/AKT signaling in regulating cell cycle progression. Inhibition of both pathways with increasing doses of U0126 and LY294002, respectively, induced accumulation in G0/G1 from the proerythroblast stage until the late basophilic/polychromatic stage (U0126 vehicle vs. 1uM p= 0.0023; LY294002 vehicle vs 1uM p=0.0389). At these later stages, ERK and PI3K/AKT inhibition led to a decrease in G0/G1 percentages, suggesting a stage-specific switch in signaling mediated cell cycle regulation, with PI3K inhibition demonstrating more potent and consistent effects (U0126 vehicle vs 1uM p=0.0406, 0.0481; LY294002 vehicle vs 1uM p=0.0003, 0.0197, 0.0086, n=3). These patterns suggest that ERK and AKT may facilitate cell cycle progression past the G0/G1 checkpoint in early erythropoiesis while inducing cell cycle exit or accumulation in G0/G1 in late erythropoiesis.
In scat, we previously characterized increased active Ras in erythroid cells and a delay in terminal erythroid differentiation with accumulation at the polychromatic stage. We therefore next sought to examine the potential mechanistic contribution of altered PI3K/AKT signaling and cell cycle progression to the differentiation delay seen in scat. Phospho-flow analyses demonstrate that scat bone marrow-derived basophilic and polychromatic erythroblasts have increased AKT activation compared to WT (p=0.0402, p=0.0559, respectively; n=4), with similar trends evident in scat spleen basophilic erythroblasts (p=0.064; n=4). These results are consistent with increased Ras activation in scat. Ex vivo EdU/PI analyses revealed that scat bone marrow-derived polychromatic erythroblasts demonstrate G0/G1 accumulation (p=0.0466) and decreased progression to S-phase (0.0414; n=6), also with similar trends in scat spleen basophilic erythroblasts (p=0.004; n=6). These results correlate with the observed differentiation delay in scat and indicate that RASA3 regulates stage-specific signaling and cell cycle progression during erythropoiesis. To study if cell cycle dysregulation in scat begins at an earlier stage of erythroid differentiation, we analyzed murine hematopoietic and erythroid progenitors as Ter119-, cKit+ cells expressing increasing levels of CD71 and found that both CD71lo and CD71med bone marrow-derived scat progenitors present with G0/G1 accumulation (p=0.0015, p=0.0073, respectively) and decreased progression to S-phase (p=0.0014, p=0.0241; n=7). This suggests a dynamic relationship between RASA3, Ras signaling, and cell cycle progression throughout early and late erythroid differentiation.
Together, these findings support the role of RASA3 as a regulator of the signaling networks governing erythropoiesis and reveal a new targetable axis in a model of inherited bone marrow failure.

Disclosures
No relevant conflicts of interest to declare.