Fyn kinase is a novel modulator of erythropoietin signaling and stress erythropoiesis.

The signaling cascade induced by the interaction of erythropoietin (EPO) with its receptor (EPO-R) is a key event of erythropoiesis. We present here data indicating that Fyn, a Src-family-kinase, participates in the EPO signaling-pathway, since Fyn-/- mice exhibit reduced Tyr-phosphorylation of EPO-R and decreased STAT5-activity. The importance of Fyn in erythropoiesis is also supported by the blunted responsiveness of Fyn-/- mice to stress erythropoiesis. Fyn-/- mouse erythroblasts adapt to reactive oxygen species (ROS) by activating the redox-related-transcription-factor Nrf2. However, since Fyn is a physiologic repressor of Nrf2, absence of Fyn resulted in persistent-activation of Nrf2 and accumulation of nonfunctional proteins. ROS-induced over-activation of Jak2-Akt-mTOR-pathway and repression of autophagy with perturbation of lysosomal-clearance were also noted. Treatment with Rapamycin, a mTOR-inhibitor and autophagy activator, ameliorates Fyn-/- mouse baseline erythropoiesis and erythropoietic response to oxidative-stress. These findings identify a novel multimodal action of Fyn in the regulation of normal and stress erythropoiesis.

Nrf2 Plays a Key Role in Erythropoiesis during Aging.

Aging is characterized by increased oxidation and reduced efficiency of cytoprotective mechanisms. Nuclear factor erythroid-2-related factor (Nrf2) is a key transcription factor, controlling the expression of multiple antioxidant proteins. Here, we show that Nrf2-/- mice displayed an age-dependent anemia, due to the combined contributions of reduced red cell lifespan and ineffective erythropoiesis, suggesting a role of Nrf2 in erythroid biology during aging. Mechanistically, we found that the expression of antioxidants during aging is mediated by activation of Nrf2 function by peroxiredoxin-2. The absence of Nrf2 resulted in persistent oxidation and overactivation of adaptive systems such as the unfolded protein response (UPR) system and autophagy in Nrf2-/- mouse erythroblasts. As Nrf2 is involved in the expression of autophagy-related proteins such as autophagy-related protein (Atg) 4-5 and p62, we found impairment of late phase of autophagy in Nrf2-/- mouse erythroblasts. The overactivation of the UPR system and impaired autophagy drove apoptosis of Nrf2-/- mouse erythroblasts via caspase-3 activation. As a proof of concept for the role of oxidation, we treated Nrf2-/- mice with astaxanthin, an antioxidant, in the form of poly (lactic-co-glycolic acid) (PLGA)-loaded nanoparticles (ATS-NPs) to improve its bioavailability. ATS-NPs ameliorated the age-dependent anemia and decreased ineffective erythropoiesis in Nrf2-/- mice. In summary, we propose that Nrf2 plays a key role in limiting age-related oxidation, ensuring erythroid maturation and growth during aging.

Megakaryocyte Cytoskeletal Proteins in Platelet Biogenesis and Diseases.

Thrombopoiesis governs the formation of blood platelets in bone marrow by converting megakaryocytes into long, branched proplatelets on which individual platelets are assembled. The megakaryocyte cytoskeleton responds to multiple microenvironmental cues, including chemical and mechanical stimuli, sustaining the platelet shedding. During the megakaryocyte's life cycle, cytoskeletal networks organize cell shape and content, connect them physically and biochemically to the bone marrow vascular niche, and enable the release of platelets into the bloodstream. While the basic building blocks of the cytoskeleton have been studied extensively, new sets of cytoskeleton regulators have emerged as critical components of the dynamic protein network that supports platelet production. Understanding how the interaction of individual molecules of the cytoskeleton governs megakaryocyte behavior is essential to improve knowledge of platelet biogenesis and develop new therapeutic strategies for inherited thrombocytopenias caused by alterations in the cytoskeletal genes.