The Role of Gpibα in Control of Megakaryocyte Reactivity to Thrombopoietin

Thrombopoietin (TPO) was first described in 1958 as a humoral factor that regulates platelet production. TPO binds to c-Mpl and stimulates the expansion and differentiation of megakaryocyte precursors and the maturation of megakaryocytes. Notably, TPO has no significant effect on the formation stage of megakaryocyte boundary membrane system and platelet release. Studies investigating GPIbα deficiency have demonstrated its substantial effects on megakaryocyte proliferation, ploidy maturation, and resultant platelet count reduction. Furthermore, our previous study demonstrated that GPIbα deficiency in mice and patients with Bernard-Soulier syndrome (BSS) results in reduced circulating TPO levels. Surprisingly, this decrease in TPO levels is not due to increased TPO clearance by platelets lacking GPIbα but rather a decrease in TPO production, which is regulated by GPIbα through the interaction with liver cells. Based on these findings, we hypothesize that augmenting exogenous TPO levels may effectively promote platelet production in GPIbα deficiency.

We first generated C57/BL6 background GPIbα deficiency mice and measured their platelet levels using an automatic blood routine counter, confirming previous findings of a lower platelet count in GPIbα deficiency mice. Subsequently, both GPIbα deficiency and wild-type mice were intraperitoneally injected with recombinant mouse TPO (rmTPO) simultaneously. Surprisingly, we observed that the GPIbα deficiency mice exhibited reduced reactivity to TPO, resulting in a significant decrease in platelet production compared to wild-type mice.

To elucidate it mechanism, we conducted comprehensive studies in vitro, isolating bone marrow-derived lin- cells from both GPIbα-deficient and wild-type mice using magnetic beads. These cells were then cultured in the presence of rmTPO, and the results revealed that GPIbα deficiency led to the inhibition of megakaryocyte cell ploidy maturation and platelet production compared to wild-type mice. Furthermore, we designed RNA small interference sequences targeting GPIbα. By conducting in vitro interference experiment using wild-type mice bone marrow-derived lin- cells with TPO, the knockdown of GPIbα resulted in inhibited megakaryocyte cell ploidy maturation and platelet production, consistent with the findings from the deficiency mice.

In addition to our prior experiments investigating the GPIbα mediated TPO generation, we have also explored the impact of GPIbα antibodies on TPO production, revealing a significant reduction in TPO levels among GPIbα-positive immune thrombocytopenia (ITP) patients. These data raise our curiosity regarding the responsiveness to TPO in ITP, particularly among GPIbα-positive ITP patients.

To verify our hypothesis, a retrospective analysis was performed to assess the response to TPO-related treatments in ITP patients, including eltrombopag, romiplostim, avatrombopag and recombinant human thrombopoietin (rhTPO). The outcomes observed in ITP patients positive for GPIbα antibodies revealed noteworthy patterns. The retrospective analysis revealed that ITP patients exhibiting GPIbα positivity displayed a worse response to thrombopoietin receptor agonists (TPO-RA) and rhTPO treatment.

To gain deeper insights into the aforementioned findings, we conducted in vitro investigations on antibody-bound megakaryocytes. Specifically, we cultured wild-type mice bone marrow-derived lin- cells in the presence of TPO, with or without anti-GPIbα antibody intervention. The antibody group exhibited inhibition of megakaryocyte ploidy maturation and platelet production compared to the negative control group.

In summary, our findings suggest that GPIbα plays a critical role in control of TPO reactivity, and its absence or interference can lead to impaired megakaryocyte ploidy maturation and platelet production. Moreover, ITP patients with anti-GPIbα antibody is associated with decreased TPO responsiveness, probably contributing to the observed poor response to TPO-based treatments. These findings have significant clinical implications, shedding light on platelet-related disorders and potential therapeutic strategies. Specifically, understanding the influence of GPIbα on megakaryocyte TPO reactivity will offer avenues for novel therapeutic interventions targeting platelet-related disorders.