Another path to ERK activation

In this issue of Blood, Kumkhaek et al demonstrate a role for the Ras-like protein MASL1 in erythroid development. MASL1 acts at least in part by stimulating the Raf mitogen-activated protein kinase kinase (MEK) extracellular signal-regulated kinase (ERK) pathway.¹ 

Ras proteins are molecular switches that activate downstream signal transduction pathways. In erythroid cells, Ras is activated by erythropoietin receptor (Epor) signaling. Epor signaling activates Ras, which binds and activates the serine-threonine kinase Raf and in turn activates the MEK-ERK mitogen-activated protein kinase cascade. MASL1 is a Ras-like protein that is selectively expressed in erythroid cells.¹  In the current study, Kumkhaek et al report that MASL1 binds Raf and activates this pathway, and that MASL1 is required to fully activate ERK in erythroid cells.²  Additionally, they show that MASL1 has a critical role in erythroid development.

The investigators employ a loss-of-function approach in primary human progenitor (CD34+) cells. In this model, they observe that knockdown of MASL1 with small interfering RNA or short hairpin RNA has a profound effect on erythroid development. MASL1-deficient erythroid progenitors fail to proliferate or to progress along the erythroid differentiation pathway. Indeed, by mid-culture, there are large differences in the composition of cell types present. The block in differentiation caused by MASL1 deficiency occurs at an early stage in erythroid development; significantly fewer MASL1-deficient progenitors express CD71 on their cell surface than normal, and almost none express glycophorin A. Notably, the developmental block is partially recapitulated by enforced expression of MASL1 mutated in its Ras-like GTPase domain, directly implicating the loss of Ras-like activity in the MASL1-deficient phenotype.

Consistent with the presence of a Ras-like domain in MASL1, MASL1-deficient erythroid progenitors cultured in erythropoietin fail to activate the Raf-MEK-ERK pathway. One caveat in interpreting this result is that it may be attributable to differences in the composition of cell types present, especially at later time points. However, failure to activate this pathway is also observed at early time points in the culture, when the percentage of erythroid cells is similar. Overall, the evidence presented by the investigators is compatible with the notion that MASL1 functions through the Raf-MEK-ERK pathway.

Ras activates the Raf-MEK-ERK pathway immediately upon Epor ligation. Epor activation recruits the adaptor Grb2, which in turn recruits the guanine nucleotide exchange factor Sos. Sos activates Ras, which recruits Raf to plasma membrane, activating the Raf-MEK-ERK cascade. Like Ras, MASL1 interacts with Raf; however, in contrast to Ras, there is little evidence that this interaction is directly regulated by Epor signaling. Rather, Epor signaling through Ras appears to positively regulate MASL1 expression. This suggests a model wherein sustained Epor stimulation increases MASL1 expression and overall ERK activity. Whether MASL1 is activated by the Epor or another receptor, or perhaps by an intracellular signal, will be interesting to establish. A related question is whether the subcellular location of MASL1 is regulated by an extracellular or intracellular signal.

Previous studies have shown that erythroid development is controlled by Ras proteins. Enforced expression of oncogenic H-Ras causes a block in erythroid development—an effect mediated by MEK-ERK activation.^3,4 N-Ras and H-Ras are dispensable for hematopoiesis.⁵ N-Ras and K-Ras have partially overlapping function,⁶  and K-Ras is required for normal fetal erythropoiesis.^6,7  The relatively mild effect of Ras loss-of-function mutations on erythropoiesis, may be explained in part by the presence of other Ras-like proteins, such as MASL1. Apart from its role at the early stages of erythroid development, it is worth noting that MASL1 expression increases during terminal erythroid maturation, and that it is well expressed in mature erythrocytes. Due to the early block in erythroid development caused by MASL1 deficiency, the current studies cannot address the potential role of MASL1 in late erythroid or reticulocyte maturation, or its mechanism of action. Finally, oncogenic Ras mutations are often identified in patients with myeloid disorders. These effects are mediated through the MEK-ERK pathway, raising the possibility that MASL1 too could be involved in these conditions.