Tuning TPO-R signaling to influence hematopoietic stem cell differentiation and inhibit essential thrombocythemia

Abstract

Thrombopoietin (TPO) and the TPO-receptor (TPO-R, or c-MPL)) are essential for hematopoietic stem cell (HSC) maintenance and megakaryocyte differentiation. Agents that can modulate TPO-R signaling are highly desirable, both experimentally and clinically. We have developed a series of surrogate protein-ligands for TPO-R, in the form of diabodies, that homodimerize the TPO-R on the cell surface in different geometries, in effect tuning downstream signaling responses. These surrogate ligands exhibit diverse pharmacological properties, inducing graded signaling outputs, from full to partial TPO agonism and antagonism, thus decoupling the dual functions of TPO/TPO-R. Using scRNA sequencing and HSC self-renewal assays we find that partial agonistic diabodies preserved the stem-like properties of cultured HSCs, but also blocked oncogenic colony formation in Essential Thrombocythemia (ET) through inverse agonism. Our data suggest that dampening downstream TPO signaling is a powerful approach not only for HSC preservation in culture, but also for inhibiting oncogenic signaling through the TPO-R.

Distinct Assemblies of Heterodimeric Cytokine Receptors Govern Stemness Programs in Leukemia

“…STAT5 signaling has been shown to be critical for stemness and self-renewal (65-70), whereas STAT1 signaling has been implicated in monocytic differentiation and macrophage maturation (50,51). Importantly, disrupting the balance between STAT5, STAT1, and STAT3 signaling can alter differentiation kinetics and cell fate in normal hematopoiesis (6,71) and in hematologic diseases (72). We postulate that different receptor assemblies alter the balance between STAT5 and STAT1 signaling.…”

Hidden pathway for cytokine receptor activation: Structural insights into a marine sponge-derived lectin that activates the thrombopoietin receptor via recognition of the fucose moiety

“…We propose that this structural difference affects the efficacies of the two lectins. A recent study demonstrated that dimeric antibodies that bridge and dimerize MPL by binding various sites near the canonical ligand-binding domain of MPL activated the receptor in distinctive ways, which realized agonist-based decoupling of HSC self-renewal and differentiation (22). Because no structural information on the 'active' receptor complex for MPL was known, the mechanistic basis of this phenomenon was elusive.…”

The thrombopoietin receptor: revisiting the master regulator of platelet production

“…Together, these new methods will provide a better understanding of the molecular mechanism of MPL activation by TPO and open avenues to the development of novel strategies for the modulation of the pathway. In an important first step, Cui and coworkers have recently discovered and characterized anti-MPL diabodies with differential effects on signaling output[ 101 ], which appear to separate the dual roles of MPL signaling; HSC self-renewal and megakaryocyte differentiation. As the development of highly specific synthetic cytokines gathers pace[ 102 ] it is tempting to speculate that additional modalities targeting the MPL ECD to alter signaling output could also be achieved.…”