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Splicing is a central RNA-based process commonly altered in human cancers; however, how spliceosomal components are co-opted during tumorigenesis remains poorly defined. Here we unravel the core splice factor SF3A3 at the nexus of a translation-based program that rewires splicing during malignant transformation. Upon MYC hyperactivation, SF3A3 levels are modulated translationally through an RNA stem-loop in an eIF3D-dependent manner. This ensures accurate splicing of mRNAs enriched for mitochondrial regulators. Altered SF3A3 translation leads to metabolic reprogramming and stem-like properties that fuel MYC tumorigenic potential in vivo. Our analysis reveals that SF3A3 protein levels predict molecular and phenotypic features of aggressive human breast cancers. These findings unveil a post-transcriptional interplay between splicing and translation that governs critical facets of MYC-driven oncogenesis.

Dynamins are highly conserved large GTPases (enzymes that hydrolyze guanosine triphosphate) involved in endocytosis and vesicle transport, and mutations in the ubiquitous and housekeeping dynamin 2 (DNM2) have been associated with thrombocytopenia in humans. To determine the role of DNM2 in thrombopoiesis, we generated Dnm2(fl/fl) Pf4-Cre mice specifically lacking DNM2 in the megakaryocyte (MK) lineage. Dnm2(fl/fl) Pf4-Cre mice had severe macrothrombocytopenia with moderately accelerated platelet clearance. Dnm2-null bone marrow MKs had altered demarcation membrane system formation in vivo due to defective endocytic pathway, and fetal liver-derived Dnm2-null MKs formed proplatelets poorly in vitro, showing that DNM2-dependent endocytosis plays a major role in MK membrane formation and thrombopoiesis. Endocytosis of the thrombopoietin receptor Mpl was impaired in Dnm2-null platelets, causing constitutive phosphorylation of the tyrosine kinase JAK2 and elevated circulating thrombopoietin levels. MK-specific DNM2 deletion severely disrupted bone marrow homeostasis, as reflected by marked expansion of hematopoietic stem and progenitor cells, MK hyperplasia, myelofibrosis, and consequent extramedullary hematopoiesis and splenomegaly. Taken together, our data demonstrate that unrestrained MK growth and proliferation results in rapid myelofibrosis and establishes a previously unrecognized role for DNM2-dependent endocytosis in megakaryopoiesis, thrombopoiesis, and bone marrow homeostasis.

Proteins of the Bin-amphiphysin-Rvs (BAR) and Fes-CIP4 homology BAR (F-BAR) families bind and deform membranes, promoting in cells tubular invaginations reminiscent of the platelet open canalicular system (OCS) and megakaryocyte (MK) demarcation membrane system (DMS). Here we investigated the role of the F-BAR protein PACSIN2 in platelets and MKs. Spinning disk laser fluorescence confocal microscopy showed that PACSIN2 co-localized with the von Willebrand factor receptor subunit GPIbα at the openings of the OCS in platelets. Endogenous and over-expressed PACSIN2 associated with anastomosing tubular structures reminiscent of the pre-DMS in mouse bone marrow MKs. Pacsin2- null mice had mild thrombocytopenia and polycythemia, MK hyperplasia and splenomegaly. Pacsin2-null mice had a bleeding disorder, as evidenced by significantly prolonged tail bleeding and delayed in vivo thrombus formation following FeCl3 vascular injury, which was normalized by injection of WT platelets. However, Pacsin2-null platelets expressed P- selectin and activated the integrin αIIbβ3 normally in response to stimulation with thrombin and the GPVI-specific collagen-related peptide. In contrast, Pacsin2-null platelets had abnormal flattened morphology, with more and narrower OCS channels on their surface, compared to WT platelets. Together, the data indicate that the F-BAR protein PACSIN2 orchestrates the intracellular membrane architecture of platelets, thereby regulating their in vivo hemostatic functions.

Abstract 3261 Megakaryocyte (MK) maturation and platelet formation are associated with extensive intracellular membrane rearrangements. However, the cellular proteins and mechanisms responsible for the formation of the platelet open canalicular system (OCS) are unknown. PACSIN 1–3 belong to the novel family of F-BAR domain-containing proteins, which bind and deform membranes, promoting tubular invaginations. Blood platelets and MKs contain the ubiquitously expressed PACSIN 2, but not PACSIN 1 or 3, as evidenced by intracellular flow cytometry and immunoblot analysis. Spinning disk laser fluorescence confocal microscopy revealed a specific association between PACSIN 2 and tubular structures in human platelets. PACSIN 2 did not colocalize with granule or lysosome markers, i.e. CD62P, CD63 and LAMP1, suggesting that it coats the OCS. In mouse MKs, PACSIN 2 was distributed through the cell body and colocalized with the membrane marker, CD61. Immunoprecipitation of PACSIN 2 from human platelet lysates pulled down filamin A (FlnA), a major platelet cytoskeletal protein. The interaction required PACSIN 2 F-BAR domain, as evidenced by in vitro binding assays using recombinant proteins. Mice lacking FlnA in the MK lineage have a severe thrombocytopenia with large platelets (Falet et al, J Exp Med 2010; Jurak Begonja et al, Blood 2011). In contrast to wild type platelets, FlnA-null platelets had diffuse PACSIN 2 staining and abnormal intracellular membrane structures, suggesting that the PACSIN 2-FlnA interaction is necessary for the integrity of the platelet intracellular membrane systems. PACSIN 2-null mice had a mild thrombocytopenia, enlarged spleens and increased spleen megakaryopoiesis. Together, the data shows that PACSIN 2 is expressed in platelets and MKs, where it specifically coats the anastomosing intracellular membrane systems in cooperation with FlnA. Disclosures: No relevant conflicts of interest to declare.

Dynamins are large and highly conserved GTPases involved in endocytosis and vesicle trafficking. Mutations K562E/del in the ubiquitous dynamin 2 (DNM2) have been associated with thrombocytopenia in humans. To determine the role of DNM2 in megakaryopoiesis we generated Dnm2fl/fl Pf4-Cre mice specifically lacking DNM2 in the megakaryocyte (MK) lineage. Dnm2fl/fl Pf4-Cre mice were viable, but had severe macrothrombocytopenia with moderately accelerated platelet clearance and prolonged bleeding due to poorly functional platelets. Dnm2-null bone marrow MKs had altered demarcation membrane system, appearing at times as a compact, narrow twisting membrane system of clathrin-coated vesicles. Fetal liver cell derived Dnm2-null MKs formed proplatelets poorly in vitro, showing that DNM2 plays a major role in MK membrane formation and thrombopoiesis. Both endogenous DNM2 and overexpressed DNM2 WT, but not DNM2 K562E/del mutants localized with the early endosome in bone marrow MKs. The endocytic pathway was disrupted in Dnm2-null MKs, as evidenced by severely reduced early endosome EEA1 and APPL1 staining and weak LysoTracker internalization. Endocytosis of the thrombopoietin (TPO) receptor Mpl was impaired in Dnm2-null platelets, causing constitutive phosphorylation of the tyrosine kinase JAK2 and elevated circulating TPO levels. MK-specific DNM2 deletion severely disrupted bone marrow homeostasis, as reflected by massive MK hyperplasia and myelofibrosis, and consequent extramedullary hematopoiesis and splenomegaly. However, additional Mpl genetic deletion failed to rescue the severe splenomegaly of Dnm2fl/fl Pf4-Cre mice, and Mpl-/- Dnm2fl/fl Pf4-Cre mice instead died at 4-5 weeks of age. Taken together, our data demonstrates that unrestrained MK growth and proliferation results in rapid myelofibrosis independently of Mpl expression and other bone marrow cell types, and establishes a previously unrecognized role for DNM2-dependent endocytosis in megakaryopoiesis, thrombopoiesis and bone marrow homeostasis. Disclosures No relevant conflicts of interest to declare.

Proteins of the Bin-amphiphysin-Rvs (BAR) and Fes-CIP4 homology BAR (F-BAR) families bind and deform membranes, promoting in cells tubular invaginations reminiscent of the platelet open canalicular system (OCS) and megakaryocyte (MK) demarcation membrane system (DMS). Here we investigated the role of the F-BAR protein PACSIN2 in platelets and MKs. Spinning disk laser fluorescence confocal microscopy showed that PACSIN2 co-localized with the von Willebrand factor receptor subunit GPIbα at the openings of the OCS in platelets. Endogenous and over-expressed PACSIN2 associated with anastomosing tubular structures reminiscent of the pre-DMS in mouse bone marrow MKs. Pacsin2-null mice had mild thrombocytopenia and polycythemia, MK hyperplasia and splenomegaly. Pacsin2-null mice had a bleeding disorder, as evidenced by significantly prolonged tail bleeding and delayed in vivo thrombus formation following FeCl3 vascular injury, which was normalized by injection of WT platelets. However, Pacsin2-null platelets expressed P-selectin and activated the integrin αIIbβ3 normally in response to stimulation with thrombin and the GPVI-specific collagen-related peptide. In contrast, Pacsin2-null platelets had abnormal flattened morphology, with more and narrower OCS channels on their surface, compared to WT platelets. Together, the data indicate that the F-BAR protein PACSIN2 orchestrates the intracellular membrane architecture of platelets, thereby regulating their in vivo hemostatic functions. Disclosures No relevant conflicts of interest to declare.