A model for studying the hemostatic consumption or destruction of platelets

Developmental Stage-Specific Manifestations of Absent TPO/c-MPL Signalling in Newborn Mice

Congenital amegakaryocytic thrombocytopaenia (CAMT) is a disorder caused by c-MPL mutations that impair thrombopoietin (TPO) signalling, resulting in a near absence of megakaryocytes (MKs). While this phenotype is consistent in adults, neonates with CAMT can present with severe thrombocytopaenia despite normal MK numbers. To investigate this, we characterized MKs and platelets in newborn c-MPL –/– mice. Liver MKs in c-MPL –/– neonates were reduced in number and size compared with wild-type (WT) age-matched MKs, and exhibited ultrastructural abnormalities not found in adult c-MPL –/– MKs. Platelet counts were lower in c-MPL –/– compared with WT mice at birth and did not increase over the first 2 weeks of life. In vivo biotinylation revealed a significant reduction in the platelet half-life of c-MPL –/– newborn mice (P2) compared with age-matched WT pups, which was not associated with ultrastructural abnormalities. Genetic deletion of the pro-apoptotic Bak did not rescue the severely reduced platelet half-life of c-MPL –/– newborn mice, suggesting that it was due to factors other than platelets entering apoptosis early. Indeed, adult GFP+ (green fluorescent protein transgenic) platelets transfused into thrombocytopenic c-MPL –/– P2 pups also had a shortened lifespan, indicating the importance of cell-extrinsic factors. In addition, neonatal platelets from WT and c-MPL –/– mice exhibited reduced P-selectin surface expression following stimulation compared with adult platelets of either genotype, and platelets from c-MPL –/– neonates exhibited reduced glycoprotein IIb/IIIa (GPIIb/IIIa) activation in response to thrombin compared with age-matched WT platelets. Taken together, our findings indicate that c-MPL deficiency is associated with abnormal maturation of neonatal MKs and developmental stage-specific defects in platelet function.

Regulation of platelet lifespan in the presence and absence of thrombopoietin signaling

Essentials We examined platelet survival in models of absent or enhanced thrombopoietin (TPO) signaling. Platelet lifespan is normal in transgenic mice with chronically enhanced TPO signaling. Mpl deficiency does not negatively affect platelet lifespan in the absence of thrombocytopenia. We conclude that TPO and its receptor Mpl are dispensable for platelet survival in adult mice.

SUMMARY

Background It is well established that thrombopoietin (TPO), acting via its receptor Mpl, is the major cytokine regulator of platelet biogenesis. The primary mechanism by which TPO signaling stimulates thrombopoiesis is via stimulation of Mpl-expressing hematopoietic progenitors; Mpl on megakaryocytes and platelets acts to control the amount of TPO available. TPO could potentially reduce platelet and/or megakaryocyte apoptosis, and therefore increase the platelet count. However, the effect of TPO receptor signaling on platelet survival is unresolved. Methods and results Here, we investigated platelet survival in mouse models of absent or enhanced TPO signaling. In the absence of thrombocytopenia, Mpl deficiency did not negatively influence platelet lifespan, and nor was platelet survival affected in transgenic mice with chronically increased TPO signaling. Conclusions We conclude that TPO and its receptor Mpl are dispensable for platelet survival in adult mice.

Aging of platelets stored for transfusion

A goal of platelet storage is to maintain the quality of platelets from the point of donation to the point of transfusion - to suspend the aging process. This effort is judged by clinical and laboratory measures with varying degrees of success. Recent work gives encouragement that platelets can be maintained ex vivo beyond the current 5 -7 day shelf life whilst maintaining their quality, as measured by posttransfusion recovery and survival. However, additional measures are needed to validate the development of technologies that may further reduce the aging of stored platelets, or enhance their hemostatic properties.

Numerical analysis of in vivo platelet consumption data from ITP patients

Background

Numerical methods have recently allowed quantitative interpretation of in vivo murine platelet consumption data in terms of values for the random destruction rate constant (RD), intrinsic lifespan (LS), and the standard deviation of ln LS (SD), as well as the platelet production rate (PR) and age distribution (AD). But application of these methods to data obtained in thrombocytopenic patients is problematic for two reasons. First, such data has in all cases been obtained with radiolabeled platelets, and uptake of the radio-isotope by long lived cells complicates the analysis. Second, inferred values of the platelet production rate (PR) and random destruction rate (RD) are difficult to interpret, since increased RD can occur either as a cause or a consequence of thrombocytopenia.

Methods

We used a numerical method to analyze in vivo platelet consumption data from a series of 41 patients with immune thrombocytopenic purpura (ITP). An additional parameter, the fraction of labeled long-lived cells (LL), was evaluated concurrently with RD, LS, and SD. To provide a basis for interpreting these values, we used an iterative interpolation process to predict their response to different pathophysiologic mechanisms. The process also generates predicted effects on the widely used immature platelet fraction (IPF).

Results

Optimal parameter value sets were identified in 76 % (31 of 41) of the data sets. 27 of 31 ITP patients showed no substantial homeostatic increase in platelet production, with the remaining 4 showing both augmented platelet consumption and a compensatory increase in PR. Up to 1/3 of the patients showed the degree of increased RD expected to result from reduced thrombopoiesis only. “Jacknife” resampling yielded CV values of <0.5 in over 75 % of the evaluable data sets. Predicted platelet age distributions indicate that interpretation of the IPF and absolute IPF (aIPF) is a complex function of platelet count. We found, counter-intuitively, that reduced PR can increase the IPF, and increased RD can reduce the aIPF.

Conclusions

Our findings support the feasibility of using numerical analysis to quantitatively interpret in vivo platelet consumption data, to identify likely etiologies of thrombocytopenias, and to assess the utility of IPF measurements in that context.

Electronic supplementary material

The online version of this article (doi:10.1186/s12878-015-0034-4) contains supplementary material, which is available to authorized users.