Mnk1 regulates megakaryocyte endomitosis and platelet production. (A) Pearson correlation analyses of the association between platelet counts (left) and MPV (right) with platelet MKNK1 mRNA expression (n = 154 healthy human donors). Light gray lines represent 95% confidence intervals. (B) Platelet counts (left) and mean platelet volume (right) in WT or Mnk1 KO mice (n > 10 mice per group, ∗P < .05). (C) Bone marrow–derived megakaryocytes from WT or Mnk1 KO mice were collected at culture day 5, and ploidy distribution of megakaryocytes was quantified (n = 3 independent experiments), ∗P_ANOVA < .05). (D) Bone marrow–derived megakaryocytes from WT or Mnk1 KO mice were collected at culture day 5. Megakaryocytes were then allowed to adhere to fibrinogen-coated plates and incubated overnight at 37°C. Megakaryocytes were then stained with phalloidin, and proplatelet formation was assessed by confocal microscopy. The right bar graph shows the number of proplatelet forming megakaryocytes (∗P < .05, n = 3 independent experiments). (E-F) Platelets were depleted from WT or Mnk1 KO mice by a single IV injection of an anti-GPIbα antibody (Emfret Analytics), leading to near-complete platelet clearance by 48 hours. Platelet counts were measured over 120 hours (eg, 5 days) by Hemavet. The (E) percentage of platelet count recovery was calculated from the platelet nadir at 48 hours, and (F) platelet clearance was calculated from the time of injection of the anti-GPIbα antibody (n = 5 mice per group, ∗P < .05; ∗∗P < .01). ANOVA, analysis of variance; NS, not significant.