Serum response factor is an essential transcription factor in megakaryocytic maturation

Abstract

IntroductionSerum response factor (Srf), a nearly ubiquitous transcription factor that is expressed in all hematopoietic cell types, is a founding member of the MCM1-agamous-deficiens-Srf (MADS) domain-containing family of transcription factors, and binds to so called CArG sites (CC/AT-rich/GG, with CCTTATATGG emerging as a major consensus sequence). 1 More than 200 CArG boxes control expression of more than 150 Srf target genes, including genes of the cytoskeleton as well as several immediateearly genes, for example the proto-oncogene c-fos, 2 and also bcl2, 3 whereby Srf participates in apoptosis, cell growth, differentiation, and cell-cycle regulation. Srf is a downstream target of many pathways; for example, the mitogen-activated protein kinase pathway that acts through the ternary complex factors (TCF) as well as Rho signaling, 4 which promotes actin polymerization. Srf plays an important role in the regulation of smooth, skeletal, and cardiac muscle genes 5-8 during development and in adult life including aging. 9,10 Srf can be activated to promote transcription in response to extracellular signals that induce its association with specific cofactors. The 2 families of Srf cofactors are the TCF proteins (Elk1, SAP1, SAP2), 11 and the myocardin family of proteins that includes myocardin, megakaryoblastic leukemia 1 (Mkl1), and Mkl2. 1,12,13 In some cell types, TCF proteins and Mkl1 compete for binding to Srf to activate or inhibit distinct transcription targets. 14,15 In Drosophila, Srf interaction with Mkl (MAL-D) promotes cytoskeletal strength during cellular migration. 16,17 In murine embryonic stem cells, Srf is crucial for actin cytoskeletal organization and focal adhesion assembly. 18 Murine embryos lacking Srf fail to form mesoderm and thus die early in development. 19 Prior work in our laboratory has focused on acute megakaryoblastic leukemia with the t(1;22) translocation, involving fusion between an RNA binding motif protein 15 (RBM15) and Mkl1. Mkl1 is known to act as a cofactor for Srf-mediated gene activation in muscle differentiation. 12,20 We have demonstrated a physiologic role for Mkl1 in megakaryopoiesis. 21 Mkl1 expression increases with normal megakaryocyte (Mk) differentiation, and promotes Mk polyploidization. Mkl1 knockout (KO) mice have normal hematopoietic stem cells and megakaryocyte-erythroid progenitors (Pre-Meg-E). However, there is a dramatic increase in the number of CD41 ϩ megakaryocytes with most of these having low (2N) ploidy. There is a significant decrease in the number of polyploid megakaryocytes and thrombocytopenia. Mkl1 requires Srf to enhance polyploidization during megakaryocytic differentiation. Based on these findings, we have examined the effects of the Mkl1 cofactor Srf in megakaryocyte development.We show, that Srf deletion specifically in the megakaryocytic lineage leads to macrothrombocytopenia, whereas bone marrow (BM) and spleen show significant accumulation of abnormal megakaryocytes. Examination of candidate Srf target genes reveals that several actin cyt...

Role of serum response factor in the pathogenesis of disease

“…Moreover, defective platelet number and cytoarchitecture were accompanied by a prolongation in bleeding time. 92 Taken together, these findings clearly establish a role for SRF in normal platelet production and function. It will be of interest to determine whether the reduction in platelets with loss in SRF has consequences for vascular remodeling following arterial injury or other platelet-associated disorders.…”

“…90 In two very recent reports, SRF was inactivated in megakaryocytes using either an inducible Mx1-Cre driver 91 or platelet factor 4 Cre. 92 Mice with deficient levels of SRF in megakaryocytes exhibit thrombocytopenia and macrothrombocytopenia with reductions in numerous cytoskeletalassociated genes. The expected reduction in actin cytoskeletal genes likely explains the decrease in the number of protoplatelets budding from megakaryocytes.…”

Whole exome sequencing identifies genetic variants in inherited thrombocytopenia with secondary qualitative function defects

“…[30][31][32] Recently, the first case of a homozygous mutation in MKL1 in a patient with severe immunodeficiency and no hematologic malignancies was reported. 33 One interesting phenotypic presentation of the affected individual was an intermittent mild thrombocytopenia with low platelet counts in whole blood of between 50x10 9 /L and 150x10 9 /L.…”

Megakaryocytes muscle in

“…Halene and colleagues examined in-depth the role of SRF in megakaryocyte maturation and platelet production through megakaryocyte-specific ablation of the SRF gene in mice. 4 Loss of SRF results in macrothrombocytopenia with prolonged bleeding times along with increased numbers of immature, dysmorphic megakaryocytes in the bone marrow and spleen. In vitro culture experiments attribute these defects in part to impaired adherence to fibronectin and reduced proplatelet formation in methylcellulose.…”