Terminally mature megakaryocytes undergo dramatic cellular reorganization to produce hundreds of virtually identical platelets. A hallmark feature of this process is the generation of an elaborate system of branched protrusions called proplatelets. We recently identified RanBP10 as a tubulin-binding protein that is concentrated along polymerized microtubules in mature megakaryocytes. RanBP10 depletion in vitro caused the disturbance of polymerized filaments.Here we study the function of RanBP10 in vivo by generating deficient mice using a gene-trap approach. Mutant mice show normal platelet counts, and fetal liverderived megakaryocytes reveal only slightly reduced proplatelet formation. However, ultrastructural analysis unveiled a significantly increased geometric axis ratio for resting platelets, and many platelets exhibited disorders in microtubule filament numbers and localization. Mutant mice showed a markedly prolonged bleeding time. Granule release, a process that depends on internal contraction of the microtubule marginal coil, also was reduced. Flow cytometry analysis revealed reduced expression of CD62P and CD63 after PAR4-peptide stimulation. These data suggest that RanBP10 plays an essential role in hemostasis and in maintaining microtubule dynamics with respect to both platelet shape and function. (Blood. 2009;114:5532-5540) IntroductionPlatelets develop from megakaryocytes (MKs), large polyploid cells localized in the bone marrow. Mature MKs rearrange their entire cytoplasm into pseudopod protrusions (designated proplatelets), a process that is driven by and dependent on microtubules (MTs). 1,2 MT filaments emanate from the cell cortex into the proplatelet tips, where they coil into a loop and later build the platelet marginal band. In wild-type platelets, this MT loop is normally coiled 8 to 12 times. 3 Ultrastructural data previously suggested that the platelet marginal bundle contains a single coiled filament, [3][4][5][6] and isolation of MT coils by platelet extraction affirmed this idea. However, the study by Patel-Hett et al 7 on living platelets revealed up to 7 free plus-ends, suggesting that several shorter MT filaments associate dynamically with one longer filament, forming a bipolar array.Each MT filament is composed of 13 protofilament heteropolymers of ␣-and ␤-tubulins. Mammalian genomes encode 6 ␣-tubulin and 5 ␤-tubulin genes. ␤1-tubulin (also referred to as class VI) is the most diverse isotype 8 and is found exclusively in cells that harbor a marginal band: MKs, platelets, and erythroblasts. 9 More than 90% of proplatelet MT filaments are composed of this ␤-tubulin isotype, 10 whose expression depends on the MK-specific transcription factor NF-E2. Genetically engineered mice that lack a functional Tubb1 gene are thrombocytopenic, and Tubb1 Ϫ/Ϫ platelets lack lentiform shape; the marginal band consists of only 2 to 3 coils, resulting in platelet spherocytosis. 11,12 Although ubiquitously expressed tubulin isotypes ␤2 and ␤5 are up-regulated, the equilibrium is shifted toward monomer...

BackgroundThrombocytopenia with absent radii syndrome is defined by bilateral radius aplasia and thrombocytopenia. Due to impaired thrombopoietin signaling there are only few bone marrow megakaryocytes and these are immature; the resulting platelet production defect improves somewhat over time. A microdeletion on chromosome 1q21 is present in all patients but is not sufficient to form thrombocytopenia with absent radii syndrome. We aimed to refine the signaling defect in this syndrome. Design and MethodsWe report an extended study of 23 pediatric and adult patients suffering from thrombocytopenia with absent radii syndrome in order to scrutinize thrombopoietin signal transduction by immunoblotting and gel electrophoretic shift assays. In addition, platelet immunotyping and reactivity were analyzed by flow cytometry. Results were correlated with clinical data including age and platelet counts. ResultsTwo distinct signaling patterns were identified. Juvenile patients showed abrogated thrombopoietin signaling (pattern #1), which is restored in adults (pattern #2). Phosphorylated Jak2 was indicative of activation of STAT1, 3 and 5, Tyk2, ERK, and Akt, showing its pivotal role in distinct thrombopoietin-dependent pathways. Jak2 cDNA was not mutated and the thrombopoietin receptor was present on platelets. All platelets of patients expressed normal levels of CD41/61, CD49b, and CD49f receptors, while CD42a/b and CD29 were slightly reduced and the fibronectin receptor CD49e markedly reduced. Lysosomal granule release in response to thrombin receptor activating peptide was diminished. ConclusionsWe show a combined defect of platelet production and function in thrombocytopenia with absent radii syndrome. The rise in platelets that most patients have during the first years of life preceded the restored thrombopoietin signaling detected at a much later age, implying that these events are uncoupled and that an unknown factor mediates the improvement of platelet production.

It is well-known that children and adolescents with obesity have increased over recent decades which in turn carries greater risk of co-morbidities and poses a preventive as well as a therapeutic challenge. Currently, there are limited recommendations available on proven methods for recording physical fitness in children and adolescents presenting with extreme obesity. In this study, twenty participants, aged 12–17 years, with a body mass index (BMI) above the 99.5th percentile, were comparatively assessed, using a correlation between their physical fitness on a bicycle (BC) and treadmill (TM) cardiopulmonary exercise testing (CPET) with a lactate diagnostic. The results of the BC and the TM were as follows: maximum heart rate (HRmax) 186.4 ± 8.6 beats per minute (bpm) vs. 190.8 ± 8.8 bpm, peak oxygen consumption (VO2peak/kg) 23.5 ± 2.9 ml/min/kg vs. 25.4 ± 3.1 ml/min/kg, and maximum lactate (Lamax) 6.4 ± 1.6 mmol/l vs. 5.6 ± 1.4 mmol/l. The values of HRmax and VO2peak/kg were significantly higher for adolescents tested on the TM. However, no significant difference was observed in either Lamax values or between the genders.    Conclusions: The higher values of HRmax and VO2peak/kg could be attributed to the activation of a higher percentage of muscle mass on the TM. Lower Lamax values on the TM suggest maximum physical exertion was not achieved. This could be due to the extreme body weight carried by the participants. Both the BC and the TM CPET could be used for assessing physical fitness in children and adolescents with extreme obesity but should not be used interchangeably. What is Known:• Currently, there are only limited recommendations available on proven methods for recording physical fitness in children and adolescents with extreme obesity available. What is New:• Cardiopulmonary exercise testing with maximum physical exertion has been shown to be feasible in children and adolescents with extreme obesity. The results obtained from this study demonstrated that both a bicycle and a treadmill can be effectively used for assessing the physical fitness levels in children and adolescents with extreme obesity.

1561 Thrombocytopenia-absent radii syndrome (TARS) is a rare congenital disorder defined by low platelet count and bilaterial aplasia of radii. Additionally, patients have perinatal eosinophilia and leukocytosis and are often anemic during the first years of life. At this time, a moderate, but enigmatic increase in platelet counts has been described, but patients remain thrombocytopenic and eventually continue to suffer from severe episodes of bleeding. Megakaryocytes, the immediate precursor cells of platelets, are scarce in the bone marrow and precursor cells fail to produce megakaryocytic colonies in response to thrombopoietin (TPO). Recently, we demonstrated that all TARS patients harbor a microdeletion on chromosome 1q21 which spans 120–200 kb, comprising 12–18 annotated genes. The deletion is also present in some unaffected parents (carriers) indicating that it is essential but not sufficient for generating the TARS phenotype. We analyzed 158 platelet counts of 33 patients over time and found that platelets increase within 2 years of life in most of our patients, but even in adult patients counts do not reach the lower norm. Thus, we performed an extended analysis of TPO signal transduction in platelets from 20 TARS patients. Overall, Jak2 kinase - despite being expressed in comparable amount - does not become phosphorylated in response to TPO when patients were below age 20, confirming our previous results also performed on young patients. Intriguingly, in platelets isolated from patients over age 20, Jak2 did become phosphorylated. As TPO activates several distinct pathways, we looked for the consequences of this bipartite TPO-responsiveness, including the activation of the alternate januskinase Tyk2, the STAT, the MAPK/ERK, and the Akt pathways. As expected, when Jak2 was not phosphorylated, Tyk2 and all downstream pathways were inactive. In contrast, in the presence of phosphorylated Jak2 (pJak2), all downstream pathways were activated, emphasizing the key role of Jak2 for TPO responsiveness. Platelets from either 20 healthy children or 11 carriers showed normal TPO signaling, excluding that the effect was due to a general age-dependence or a mere consequence of the microdeletion. Densitometric analyses confirmed our overall visual results. Expression levels of the TPO-receptor c-Mpl was not altered in 2 young and 2 adult patients compared to carriers, healthy children and adult controls, arguing against a compensatory upregulation in older patients. Furthermore, we sequenced all coding regions of Jak2 mRNA derived from patient-derived lymphoblastic cell lines (LCL) of one young and one adult patient and could not find any mutations. As bone marrow biopsies are typically not performed, changes in bone marrow cellularity or composition are not directly accessible. Recently, the immature platelet fraction (IPF) has been considered a surrogate marker for megakaryopoiesis. Interestingly, while there was no correlation between platelet count and IPF in 16 patients with TARS, we found a negative correlation between IPF with age. In 9 pediatric patients IPF was elevated (4.6%) compared to the median of 100 pediatric controls (2.7%), while in 7 adult TARS patients the mean IPF was 2.4%. These data provide circumstantial evidence that changes in megakaryopoiesis might drive the change in platelet biogenesis and TPO signaling. Plasma levels of stromal derived factor 1, a chemokine that contributes to restore platelet production in the absence of functional TPO signaling, were within the normal range in 6 patients with TARS. Real-time analysis of mRNA expression in LCL of genes within the microdeleted region indicates comparable expression in 2 unaffected parents with 2 controls, while 3 patients and 2 carriers showed the expected reduced expression. This includes the expression of PIAS3, a negative regulator of the Jak-STAT pathway. PIAS3 protein level, however, was normal in platelet lysates of TARS patients, making a key function for thrombocytopenia in TARS unlikely. Taken together, our data show an unexpected age-dependent change in TPO-signaling in platelets of TARS patients. As this change occurs much later than the amelioration of platelet counts, we suggest that an unknown factor influences platelet biogenesis during childhood. Disclosures: No relevant conflicts of interest to declare.

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