Genetic studies reveal an unexpected negative regulatory role for Jak2 in thrombopoiesis

Abstract

• Jak2 deletion in PLTs andMKs leads to thrombocytosis due to dysregulated TPO turnover.• Jak2 loss in PLTs/MKs induces non-autonomous expansion of stem/progenitors, and specifically of MK-primed hematopoietic stem cells (HSCs).JAK inhibitor treatment is limited by the variable development of anemia and thrombocytopenia thought to be due to on-target JAK2 inhibition. We evaluated the impact of Jak2 deletion in platelets (PLTs) and megakaryocytes (MKs) on blood counts, stem/ progenitor cells, and Jak-Stat signaling. Pf4-Cre-mediated Jak2 deletion in PLTs and MKs did not compromise PLT formation but caused thrombocytosis, and resulted in expansion of MK progenitors and Lin 2 Sca1 1 Kit 1 cells. Serum thrombopoietin (TPO) was maintained at normal levels in Pf4-Cre-positive Jak2 f/f mice, consistent with reduced internalization/ turnover by Jak2-deficient PLTs. These data demonstrate that Jak2 in terminal megakaryopoiesis is not required for PLT production, and that Jak2 loss in PLTs and MKs results in non-autonomous expansion of stem/progenitors and of MKs and PLTs via dysregulated TPO turnover. This suggests that the thrombocytopenia frequently seen with JAK inhibitor treatment is not due to JAK2 inhibition in PLTs and MKs, but rather due to JAK2 inhibition in stem/progenitor cells. (Blood. 2014;124(14):2280-2284

Sympathetic stimulation facilitates thrombopoiesis by promoting megakaryocyte adhesion, migration, and proplatelet formation

“…42 Additionally, adrenergic stresses were found to induce a more apparent increase in the platelet counts in mice with the c-Mpl homozygous mutation, probably because TPO/c-Mpl does not work on terminal MK maturation and even negatively regulate platelet shedding through Janus kinase-signal transducer and activator of transcription signaling activation. [43][44][45] This finding also suggests that the sympathetic stimulation may play an important role in maintaining thrombopoiesis when the TPO/c-Mpl-associated pathway is defective.…”

Presence of atypical thrombopoietin receptor (MPL) mutations in triple-negative essential thrombocythemia patients

“…42 This has been well demonstrated in 3 murine models. [43][44][45] Here, we clearly demonstrate that the thrombocytosis induced by MPLS204P is not due to a defect in cell trafficking of the receptor. In contrast, MPLS204P is a weak gain-of-function mutant in signaling, inducing constitutive STAT activation using sensitive approaches and an increased and more prolonged ligand-induced STAT phosphorylation than MPLWT.…”

Dynamin 2–dependent endocytosis is required for normal megakaryocyte development in mice

“…[50][51][52] Recent studies confirmed the specificity of the Pf4-Cre model to mature MKs and platelets using Mpl and JAK2 deletion or JAK2-V617F expression. 24,[53][54][55] Accordingly, Dnm2 fl/fl Pf4-Cre mice had normal erythrocyte counts and volumes, and granulocyte and monocyte counts were increased only by 30%, whereas these parameters are severely altered in JAK2-V617F knockin mouse models. [56][57][58][59][60] DNM2 deletion in HSPCs is expected to severely alter erythropoiesis, because ubiquitous Dnm2 deletion is lethal in mice before embryonic day 8.5, 8 a stage in which first erythrocytes circulate and the heterozygous DNM2 loss-of-function mutation V235G induced by chemical mutagenesis is associated with iron deficiency anemia due to impaired transferrin uptake in mice.…”