MAL: not just a leukemia inducer

The MAL gene has been described as an inducer of leukemia. In this issue of Blood, Gilles and colleagues report MAL's new role in proplatelet formation.¹ 

The megakaryoblastic leukemia 1 (MAL) gene, which encodes a transcriptional coactivator of serum response factor, has been widely studied in the context of its translocation and fusion with the gene encoding one-twenty-two (a chromosome 22 encoded protein fused in 3′ with RBM15(OTT1) located on chromosome 1), leading to acute megakaryoblastic leukemia (AMKL, M7).^2,3  As described in this issue of Blood, Gilles et al show that MAL controls megakaryocyte migration and proplatelet formation.¹  First, the authors demonstrated that MAL expression increases during the late differentiation steps of neonate and adult human megakaryopoiesis. The subcellular localization of MAL is regulated through its association with globular actin, and the modification of actin by the Rho pathway results in the nuclear accumulation of MAL.⁴  In accordance, following Rho GTPases activation by adhesion on collagen I or convulxin, MAL localized into the nucleus. To examine the significance of this cellular expression and localization, MAL was knocked down in megakaryocyte progenitors, leading to a reduction in the percentage of cells forming filopodia, lamellipodia, and stress fibers after adhesion. MAL down-regulation resulted in dysmorphic megakaryocytes with disorganized demarcation membranes and alpha granules heterogeneously scattered in the cytoplasm. Considering that these properties are known to affect proplatelet formation, the investigators next measured proplatelet generation, establishing a function for MAL in this process. This finding further supports a role for the actomyosin cytoskeleton in proplatelet formation, in addition to the well-established importance of the microtubules in proplatelet formation.⁵ 

To explore mechanisms associated with effects of MAL on proplatelet formation, the authors examined gene expression, which showed a decrease in the levels of MMP-9 and MYL9 expression in MAL–down-regulated cells. Luciferase assays in HEK293T cells and chromatin immunoprecipitation in primary megakaryocytes demonstrated that the MAL/SRF complex directly regulates myosin light chain 9 (MYL9) and MMP-9 in vitro. The authors focused on these 2 factors, among others regulated by acute myeloid leukemia (AML), because of their known effects on cell migration. Indeed, megakaryocytes migration in response to SDF-1 was decreased following MAL knockdown, potentially implicating MMP-9 as a mediator of the effect of MAL on migration. Phosphorylation of MYL9 has already been reported to activate MYH9 and to negatively regulate proplatelet formation in normal MK.⁶  Here, Gilles et al showed by application of MYL9 shRNA that MYL9 is involved in proplatelet formation.

In the future, it would be interesting to address differences between mouse and human models, in light of the thrombocytopenia and increase in proplatelet-forming megakaryocytes seen in MYH9-deficient mice. Also, the authors do not rule out the possibility of involvement in proplatelet formation of other genes identified as regulated by MAL. For instance, the focus in the study at hand has been only on displayed genes known to have a key role in cytoskeleton organization. Future studies could explore the functional significance for proplatelet formation of other genes that are deregulated as a result of MAL down-regulation or overexpression. Finally, also of interest is the possibility that alpha granules heterogeneously scattered in the cytoplasm of AML-deregulated cells bear heterogeneous contents, a phenomenon that has been recently explored in other studies.