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In this issue of Blood, Halene and colleagues reveal an essential function of the transcription factor SRF during megakaryocyte maturation and platelet formation. The spectrum of SRF-regulated genes overlaps with SRF targets in muscle cells, including those involved in actin cytoskeletal dynamics.

Serum response factor (SRF) is a highly conserved transcription factor that binds to a DNA motif known as serum response element.¹  Although ubiquitously expressed in adult tissues, SRF expression during embryonic development is largely restricted to cardiac and skeletal muscles.²  SRF plays a critical role in cardiogenesis and is a major determinant of cardiomyocyte differentiation.²  With low intrinsic transcriptional activity itself, SRF recruits cofactors to confer strong transactivation potential in a cell context-specific manner. SRF associates with 2 classes of coactivators, 1 of which is the myocardin family of proteins that includes myocardin, and myocardin-related transcription factors MRTF-A (also known as megakaryoblastic leukemia 1, or MKL1) and MRTF-B (or MKL2).³  SRF-MKL target genes encode proteins involved in muscle-specific and contractile functions, and actin microfilament dynamics and cell motility.³ 

SRF regulates many cellular processes in diverse cell types but its role in hematopoiesis is underexplored. 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. Immunofluorescence studies showed that SRF-deficient megakaryocytes and platelets have intact tubulin bundles but abnormal stress fiber formation with disorganized filamental actin. Transmission electron microscopy further revealed an abnormal morphology of giant platelets, and lack of a uniform demarcation membrane system in the megakaryocytes. Furthermore, transcriptome analysis uncovered reduced expression in SRF-deficient megakaryocytes of numerous genes controlling actin cytoskeleton.

Among the most down-regulated genes in SRF-deficient megakaryocytes are the actin cross-linking protein Filamin A, the actin-binding protein Coronin 1a, and the actin filament-associated regulatory protein Calponin,⁴  as well as genes involved in cell motility.^5,6  Notably, SRF also regulates many of these genes in other cell types, such as muscle cells.³  This similarity suggests that SRF assumes similar functions in diverse cell lineages via similar mechanisms and also underscores the importance of actin cytoskeletal organization and dynamics in megakaryopoiesis and thrombopoiesis.⁷  This study, along with several others, contrasts with the original view that SRF is a serum-stimulated transcription factor that regulates “immediate early” growth genes.^3,8  Rather, a large fraction of SRF target genes are in fact structural genes related to actin biogenesis and contractile functions.^3,8  Furthermore, mouse genetic studies in multiple cell types suggest that SRF is dispensable for cell proliferation, but is instead crucial for the actin cytoskeleton, cell migration and contraction, as well as differentiation.^3,8 

The phenotypic consequences of SRF loss in megakaryocytes are likely an underestimate due to the late and incomplete conditional excision of SRF in the megakaryocytic lineage. However, the intriguing results from this report warrant further studies in earlier megakaryocyte development and additional hematopoietic lineages using appropriate conditional transgenes.

A role for SRF during megakaryopoiesis is not unexpected in light of 2 previous reports showing that 1 of its cofactors, MKL1, plays important roles in megakaryopoiesis.⁹  MKL1 overexpression promotes megakaryocytic differentiation of human CD34⁺ cells cultured in thrombopoietin.⁹  The effects of MKL1 overexpression are lost when SRF is depleted, suggesting that MKL1 function is dependent on SRF.⁹  Moreover, knockdown of MKL1 in human progenitor cells and knockout of MKL1 in mice results in dysmorphic megakaryocytes with reduced proplatelet formation.^6,9  Not surprisingly, loss of MKL1 confers a less profound perturbation in megakaryocyte maturation compared with SRF deficiency. SRF recruits several cofactors, including MKL2 that might compensate for the loss of MKL1. In addition, SRF recruits members of the Ets family of coactivators, such as the ternary complex factors, to the serum response elements.³  Assigning individual SRF cofactors to specific SRF targets requires further studies.

SRF activity is highly regulated and vital for many developmental processes.¹⁰  Importantly, a growing number of human diseases is associated with changes in the activity of SRF and its cofactors.^3,8,10  The t(1;22) chromosomal translocation found in a subset of acute megakaryoblastic leukemia (AMKL) in children results in the in-frame fusion of the OTT transcription factor to MKL1 (also known as OTT-MAL). Expression of the oncogenic OTT-MKL1 gene from the endogenous OTT locus in mice leads to aberrant differentiation of hematopoietic stem/progenitor cells with a bias toward the megakaryocytic lineage.¹¹  Progression to full-blown AMKL requires the cooperativity with the activated thrombopoietin receptor.¹¹  The megakaryoblast proliferation also depends on the interaction between the OTT portion of the fusion protein and a component of the canonical Notch pathway.¹¹  However, whether the MKL1 portion of the oncogene plays a role in megakaryocyte differentiation and/or blast transformation and whether it is mediated by SRF during the pathogenesis of AMKL remains to be investigated.