Sp1/Sp3 transcription factors regulate hallmarks of megakaryocyte maturation, and platelet formation and function

Sp1 and Sp3 belong to the Specificity proteins (Sp)/Krüppel-like transcription factorfamily. They are closely related, ubiquitously expressed and recognize G-rich DNAmotifs. They are thought to regulate generic processes such as cell cycle and growthcontrol, metabolic pathways and apoptosis. Ablation of Sp1 or Sp3 in mice is lethal, andcombined haploinsufficiency results in hematopoietic defects during the fetal stages.Here, we show that in adult mice conditional ablation of either Sp1 or Sp3 has minimalimpact on hematopoiesis, while the simultaneous loss of Sp1 and Sp3 results in severemacrothrombocytopenia and platelet dysfunction. We employed flow cytometry, cellculture and electron microscopy and show that although megakaryocyte numbers arenormal in bone marrow and spleen, they display a less compact demarcation membranesystem and a striking inability to form proplatelets. Through megakaryocytetranscriptomics and platelet proteomics we identified several cytoskeleton-relatedproteins and downstream effector kinases, including Mylk, that were downregulatedupon Sp1/Sp3 depletion, providing an explanation for the observed defects inmegakaryopoiesis. We show that Mylk is required for proplatelet formation andstabilization and for ITAM-receptor mediated platelet aggregation. Our data highlightsthe specific vs generic role of these ubiquitous transcription factors in the highlyspecialized megakaryocytic lineage.