Integrated Safety Update of Abrocitinib in 3802 Patients with Moderate-to-Severe Atopic Dermatitis: Data from More than 5200 Patient-Years with Up to 4 Years of Exposure.

BACKGROUND: Abrocitinib, an oral, once-daily, Janus kinase 1-selective inhibitor, is efficacious in moderate-to-severe atopic dermatitis with a manageable long-term safety profile. OBJECTIVE: We aimed to provide updated integrated long-term safety results for abrocitinib from available data accrued up to a maximum of almost 4 years in patients with moderate-to-severe atopic dermatitis from the JADE clinical development program. METHODS: Analysis included 3802 patients (exposure: 5213.9 patient-years) from the phase II monotherapy study (NCT02780167) and the phase III studies JADE MONO-1 (NCT03349060), JADE MONO-2 (NCT03575871), JADE TEEN (NCT03796676), JADE COMPARE (NCT03720470), JADE DARE (NCT04345367; 200 mg only), JADE REGIMEN (NCT03627767), and JADE EXTEND (NCT03422822; data cutoff 25 September, 2021). Data from patients receiving one or more doses of abrocitinib 200 mg or 100 mg were pooled in a consistent-dose cohort (patients were allocated to receive the same abrocitinib dose throughout exposure in the qualifying parent study and/or long-term study) or a variable-dose cohort (patients received open-label abrocitinib 200 mg; responders were randomized to abrocitinib 200 mg, 100 mg, or placebo, and could then receive abrocitinib 200 mg plus topical corticosteroids as rescue therapy). Incidence rates of adverse events of special interest were assessed. Cox regression analysis of risk factors for herpes zoster and serious infections was performed. RESULTS: Overall, this safety analysis of long-term data up to a maximum of ~ 4 years of abrocitinib exposure does not indicate any changes from the previously reported risk profile. The most frequent serious infections (per Medical Dictionary for Regulatory Activities preferred term) with consistent-dose abrocitinib 200 mg and 100 mg were herpes zoster (0.5% and 0.2%), pneumonia (0.2% with either dose), and herpes simplex (0.1% with either dose). Risk factors for herpes zoster were a history of herpes zoster, abrocitinib 200-mg dose, age ≥ 65 years, absolute lymphocyte count < 1 × 103/mm3 before the event, and residing in Asia. For serious infections, > 100 kg body weight was a risk factor. Incidence rate/100 patient-years (95% confidence interval) with the consistent abrocitinib 200-mg and 100-mg dose combined was higher in older (aged ≥ 65 years) patients versus younger (aged 18 to < 65 years) patients for serious adverse events (17.6 [11.7‒25.4] vs 6.7 [5.8‒7.8]), malignancy excluding non-melanoma skin cancer (2.4 [0.6‒6.0] vs 0.1 [0.0‒0.4]), non-melanoma skin cancer (2.4 [0.6‒6.1] vs 0.2 [0.1‒0.4]), lymphopenia (3.5 [1.3‒7.6] vs 0.1 [0.0‒0.3]), and venous thromboembolism (1.7 [0.4‒5.1] vs 0.1 [0.0‒0.3]). Incident rate/100 patient-years (95% confidence interval) of non-melanoma skin cancer with the consistent abrocitinib 200-mg and 100-mg dose combined was higher in current/former smokers (0.9 [0.4‒1.6]) vs never-smokers (0.0 [0.0‒0.1]). CONCLUSIONS: This safety update showed a consistent profile for abrocitinib with no new safety signals and continues to support that abrocitinib has a manageable long-term safety profile in patients with moderate-to-severe atopic dermatitis. Risk of specific adverse events was higher in certain patient populations, especially those aged ≥ 65 years. [Video abstract available.] CLINICAL TRIAL REGISTRATION: NCT02780167; study start date: April, 2016; primary completion date: March, 2017; study completion date: April, 2017. NCT03349060; study start date: 7 December, 2017; study completion date: 26 March, 2019. NCT03575871; study start date: 29 June, 2018; study completion date: 13 August, 2019. NCT03720470; study start date: 29 October, 2018; primary completion date: 27 December, 2019; study completion date: 6 March, 2020. NCT03796676; study start date: 18 February, 2019; study completion date: 8 April, 2020. NCT03627767; study start date: 11 June, 2018; primary completion date: 2 September, 2020; study completion date: 7 October, 2020. NCT04345367; study start date: 11 June, 2020; primary completion date: 16 December, 2020; study completion date: 13 July, 2021. NCT03422822; study start date: 8 March, 2018; study completion date: ongoing (estimated completion date: 31 January, 2026).

Abrocitinib is an approved treatment for people with moderate or severe atopic dermatitis, also known as AD or atopic eczema. Abrocitinib is a tablet that is taken by mouth once a day. This safety analysis looked at the side effects of treatment in a large group of adults and adolescents with moderate or severe AD who took abrocitinib up to a maximum of almost 4 years. This analysis also looked at which people were more likely to have certain side effects after taking abrocitinib. The results from this analysis were similar to those of previous safety analyses with abrocitinib, with no new side effects. Infections such as shingles, pneumonia, or herpes simplex can occur during treatment with abrocitinib. Shingles was more likely to occur in people who previously had shingles before taking abrocitinib, or who took the higher dose of abrocitinib (200 mg), or were 65 years of age or older, or had certain blood test results, or lived in Asia. People who are 65 years of age or older and took abrocitinib were more likely to develop some types of cancer, have certain abnormal blood test results, or develop blood clots in the veins than people with AD who were younger and took abrocitinib. Current or former smokers with AD who took abrocitinib were more likely to develop skin cancer (but not melanoma) than people with AD who took abrocitinib but have never smoked. This analysis further shows that abrocitinib had manageable safety in patients with moderate-to-severe AD. Video abstract: Integrated safety update of abrocitinib in 3802 patients with moderate-to-severe atopic dermatitis: data from more than 5200 patient-years with up to 4 years of exposure (MP4 63720 KB).

PACSIN2 regulates platelet integrin ß1 hemostatic function.

BACKGROUND: Upon vessel injury, platelets adhere to exposed matrix constituents via specific membrane receptors, including the von Willebrand factor receptor glycoprotein (GP)Ib-IX-V complex and integrins ß1 and ß3. In platelets, the Fes/CIP4-homology Bin-Amphiphysin-Rvs protein PACSIN2 associates with the cytoskeletal and scaffolding protein filamin A (FlnA), linking GPIbα and integrins to the cytoskeleton. OBJECTIVES: Here we investigated the role of PACSIN2 in platelet function. METHODS: Platelet parameters were evaluated in mice lacking PACSIN2 and platelet integrin ß1. RESULTS: Pacsin2-/- mice displayed mild thrombocytopenia, prolonged bleeding time, and delayed thrombus formation in a ferric chloride-mediated carotid artery injury model, which was normalized by injection of control platelets. Pacsin2-/- platelets formed unstable thrombi that embolized abruptly in a laser-induced cremaster muscle injury model. Pacsin2-/- platelets had hyperactive integrin ß1, as evidenced by increased spreading onto surfaces coated with the collagen receptor α2ß1-specific peptide GFOGER and increased binding of the antibody 9EG7 directed against active integrin ß1. By contrast, Pacsin2-/- platelets had normal integrin αIIbß3 function and expressed P-selectin normally following stimulation through the collagen receptor GPVI or with thrombin. Deletion of platelet integrin ß1 in Pacsin2-/- mice normalized platelet count, hemostasis, and thrombus formation. A PACSIN2 peptide mimicking the FlnA-binding site mediated the pull-down of a FlnA rod 2 construct by integrin ß7, a model for integrin ß-subunits. CONCLUSIONS: Pacsin2-/- mice displayed severe thrombus formation defects due to hyperactive platelet integrin ß1. The data suggest that PACSIN2 binding to FlnA negatively regulates platelet integrin ß1 hemostatic function.

Endocytosis of the thrombopoietin receptor Mpl regulates megakaryocyte and erythroid maturation in mice.

Dnm2fl/fl Pf4-Cre (Dnm2Plt-/- ) mice lacking the endocytic GTPase dynamin 2 (DNM2) in platelets and megakaryocytes (MKs) develop hallmarks of myelofibrosis. At the cellular level, the tyrosine kinase JAK2 is constitutively active but decreased in expression in Dnm2Plt-/- platelets. Additionally, Dnm2Plt-/- platelets cannot endocytose the thrombopoietin (TPO) receptor Mpl, leading to elevated circulating TPO levels. Here, we assessed whether the hyperproliferative phenotype of Dnm2Plt-/- mice was due to JAK2 constitutive activation or to elevated circulating TPO levels. In unstimulated Dnm2Plt-/- platelets, STAT3 and, to a lower extent, STAT5 were phosphorylated, but their phosphorylation was slowed and diminished upon TPO stimulation. We further crossed Dnm2Plt-/- mice in the Mpl-/- background to generate Mpl-/-Dnm2Plt-/- mice lacking Mpl ubiquitously and DNM2 in platelets and MKs. Mpl-/- Dnm2Plt-/- platelets had severely reduced JAK2 and STAT3 but normal STAT5 expression. Mpl-/- Dnm2Plt-/- mice had severely reduced bone marrow MK and hematopoietic stem and progenitor cell numbers. Additionally, Mpl-/- Dnm2Plt-/- mice had severe erythroblast (EB) maturation defects, decreased expression of hemoglobin and heme homeostasis genes and increased expression of ribosome biogenesis and protein translation genes in spleen EBs, and developed anemia with grossly elevated plasma erythropoietin (EPO) levels, leading to early fatality by postnatal day 25. Mpl-/- Dnm2Plt+/+ mice had impaired EB development at three weeks of age, which normalized with adulthood. Together, the data shows that DNM2-dependent Mpl-mediated endocytosis in platelets and MKs is required for steady-state hematopoiesis and provides novel insights into a developmentally controlled role for Mpl in normal erythropoiesis, regulating hemoglobin and heme production.

Novel GNE Gene Variants Associated with Severe Congenital Thrombocytopenia and Platelet Sialylation Defect.

The GNE gene encodes an enzyme that initiates and regulates the biosynthesis of N-acetylneuraminic acid, a precursor of sialic acids. GNE mutations are classically associated with Nonaka myopathy and sialuria, following an autosomal recessive and autosomal dominant inheritance pattern. Reports show that single GNE variants cause severe thrombocytopenia without muscle weakness. Using panel sequencing, we identified two novel compound heterozygous variants in GNE in a young girl with life-threatening bleedings, severe congenital thrombocytopenia, and a platelet secretion defect. Both variants are located in the nucleotide-binding site of the N-acetylmannosamin kinase domain of GNE. Lectin array showed decreased α-2,3-sialylation on platelets, consistent with loss of sialic acid synthesis and indicative of rapid platelet clearance. Hematopoietic stem cell transplantation (HSCT) normalized platelet counts. This is the first report of an HSCT in a patient with an inherited GNE defect leading to normal platelet counts.

Integrated Safety Analysis of Abrocitinib for the Treatment of Moderate-to-Severe Atopic Dermatitis From the Phase II and Phase III Clinical Trial Program.

BACKGROUND: Pivotal phase III studies demonstrated that abrocitinib, an oral, once-daily, JAK1-selective inhibitor, is effective treatment for moderate-to-severe atopic dermatitis (AD) as monotherapy and in combination with topical therapy. OBJECTIVE: The aim of this study was to evaluate the long-term safety of abrocitinib 200 mg and 100 mg in an integrated analysis of a phase IIb study, four phase III studies, and one long-term extension study. METHODS: Two cohorts were analyzed: a placebo-controlled cohort from 12- to 16-week studies and an all-abrocitinib cohort including patients who received one or more abrocitinib doses. Adverse events (AEs) of interest and laboratory data are reported. RESULTS: Total exposure in the all-abrocitinib cohort (n = 2856) was 1614 patient-years (PY); exposure was ≥ 24 weeks in 1248 patients and ≥ 48 weeks in 606 (maximum 108 weeks). In the placebo-controlled cohort (n = 1540), dose-related AEs (200 mg, 100 mg, placebo) were nausea (14.6%, 6.1%, 2.0%), headache (7.8%, 5.9%, 3.5%), and acne (4.7%, 1.6%, 0%). Platelet count was reduced transiently in a dose-dependent manner; 2/2718 patients (200-mg group) had confirmed platelet counts of < 50 × 103/mm3 at week 4. Incidence rates (IRs) were 2.33/100PY and 2.65/100 PY for serious infection, 4.34/100PY and 2.04/100PY for herpes zoster, and 11.83/100PY and 8.73/100PY for herpes simplex in the 200-mg and 100-mg groups, respectively. IRs for nonmelanoma skin cancer, other malignancies, and major adverse cardiovascular events were < 0.5/100PY for both doses. Five venous thromboembolism events occurred (IR 0.30/100PY), all in the 200-mg group. There were three deaths due to gastric carcinoma (diagnosed at day 43), sudden death, and COVID-19. CONCLUSION: Abrocitinib, with proper patient and dose selection, has a manageable tolerability and longer-term safety profile appropriate for long-term use in patients with moderate-to-severe AD. TRIAL REGISTRIES: ClinicalTrials.gov: NCT02780167, NCT03349060, NCT03575871, NCT03720470, NCT03627767, NCT03422822.

Immune cells surveil aberrantly sialylated O-glycans on megakaryocytes to regulate platelet count.

Immune thrombocytopenia (ITP) is a platelet disorder. Pediatric and adult ITP have been associated with sialic acid alterations, but the pathophysiology of ITP remains elusive, and ITP is often a diagnosis of exclusion. Our analysis of pediatric ITP plasma samples showed increased anti-Thomsen-Friedenreich antigen (TF antigen) antibody representation, suggesting increased exposure of the typically sialylated and cryptic TF antigen in these patients. The O-glycan sialyltransferase St3gal1 adds sialic acid specifically on the TF antigen. To understand if TF antigen exposure associates with thrombocytopenia, we generated a mouse model with targeted deletion of St3gal1 in megakaryocytes (MK) (St3gal1MK-/-). TF antigen exposure was restricted to MKs and resulted in thrombocytopenia. Deletion of Jak3 in St3gal1MK-/- mice normalized platelet counts implicating involvement of immune cells. Interferon-producing Siglec H-positive bone marrow (BM) immune cells engaged with O-glycan sialic acid moieties to regulate type I interferon secretion and platelet release (thrombopoiesis), as evidenced by partially normalized platelet count following inhibition of interferon and Siglec H receptors. Single-cell RNA-sequencing determined that TF antigen exposure by MKs primed St3gal1MK-/- BM immune cells to release type I interferon. Single-cell RNA-sequencing further revealed a new population of immune cells with a plasmacytoid dendritic cell-like signature and concomitant upregulation of the immunoglobulin rearrangement gene transcripts Igkc and Ighm, suggesting additional immune regulatory mechanisms. Thus, aberrant TF antigen moieties, often found in pathological conditions, regulate immune cells and thrombopoiesis in the BM, leading to reduced platelet count.

Increased B4GALT1 expression is associated with platelet surface galactosylation and thrombopoietin plasma levels in MPNs.

Aberrant megakaryopoiesis is a hallmark of the myeloproliferative neoplasms (MPNs), a group of clonal hematological malignancies originating from hematopoietic stem cells, leading to an increase in mature blood cells in the peripheral blood. Sialylated derivatives of the glycan structure ß4-N-acetyllactosamine (Galß1,4GlcNAc or type-2 LacNAc, hereafter referred to as LacNAc) regulate platelet life span, hepatic thrombopoietin (TPO) production, and thrombopoiesis. We found increased TPO plasma levels in MPNs with high allele burden of the mutated clones. Remarkably, platelets isolated from MPNs had a significant increase in LacNAc expression that correlated with the high allele burden regardless of the underlying identified mutation. Megakaryocytes derived in vitro from these patients showed an increased expression of the B4GALT1 gene encoding ß-1,4-galactosyltransferase 1 (ß4GalT1). Consistently, megakaryocytes from MPN showed increased LacNAc expression relative to healthy controls, which was counteracted by the treatment with a Janus kinase 1/2 inhibitor. Altered expression of B4GALT1 in mutant megakaryocytes can lead to the production of platelets with aberrant galactosylation, which in turn promote hepatic TPO synthesis regardless of platelet mass. Our findings provide a new paradigm for understanding aberrant megakaryopoiesis in MPNs and identify ß4GalT1 as a potential actionable target for therapy.

Efficiently Restored Thrombopoietin Production by Ashwell-Morell Receptor and IL-6R Induced Janus Kinase 2/Signal Transducer and Activator of Transcription Signaling Early After Partial Hepatectomy.

BACKGROUND AND AIMS: Thrombocytopenia has been described in most patients with acute and chronic liver failure. Decreased platelet production and decreased half-life of platelets might be a consequence of low levels of thrombopoietin (TPO) in these patients. Platelet production is tightly regulated to avoid bleeding complications after vessel injury and can be enhanced under elevated platelet destruction as observed in liver disease. Thrombopoietin (TPO) is the primary regulator of platelet biogenesis and supports proliferation and differentiation of megakaryocytes. APPROACH AND RESULTS: Recent work provided evidence for the control of TPO mRNA expression in liver and bone marrow (BM) by scanning circulating platelets. The Ashwell-Morell receptor (AMR) was identified to bind desialylated platelets to regulate hepatic thrombopoietin (TPO) production by Janus kinase (JAK2)/signal transducer and activator of transcription (STAT3) activation. Two-thirds partial hepatectomy (PHx) was performed in mice. Platelet activation and clearance by AMR/JAK2/STAT3 signaling and TPO production were analyzed at different time points after PHx. Here, we demonstrate that PHx in mice led to thrombocytopenia and platelet activation defects leading to bleeding complications, but unaltered arterial thrombosis, in these mice. Platelet counts were rapidly restored by up-regulation and crosstalk of the AMR and the IL-6 receptor (IL-6R) to induce JAK2-STAT3-TPO activation in the liver, accompanied by an increased number of megakaryocytes in spleen and BM before liver was completely regenerated. CONCLUSIONS: The AMR/IL-6R-STAT3-TPO signaling pathway is an acute-phase response to liver injury to reconstitute hemostasis. Bleeding complications were attributable to thrombocytopenia and platelet defects induced by elevated PGI2 , NO, and bile acid plasma levels early after PHx that might also be causative for the high mortality in patients with liver disease.

Predictive modeling of complex ABO glycan phenotypes by lectin microarrays.

Serological classification of individuals as A, B, O, or AB is a mainstay of blood banking. ABO blood groups or ABH antigens, in addition to other surface glycans, act as unique red blood cell (RBC) signatures and direct immune responses. ABO subgroups present as weakened, mixed field, or unexpected reactivity with serological reagents, but specific designations remain complex. Lectins detect glycan motifs with some recognizing ABH antigens. We evaluated a 45-probe lectin microarray to rapidly analyze ABO blood groups and associated unique glycan signatures within complex biological samples on RBC surface glycoproteins. RBC membrane glycoproteins were prepared from donor RBCs, n = 20 for each blood group. ABO blood group was distinguishable by lectin array, including variations in ABH antigen expression not observed with serology. Principal component analysis highlighted broad ABO blood group clusters with unexpected high and low antigen expression and variations were confirmed with ABH antibody immunoblotting. Using a subset of lectins provided an accurate method to predict an ABO serological phenotype. Lectin microarray highlighted the importance of ABO localization on glycoproteins and glycolipids and pointed to increased glycocalyx complexity associated with the expression of A and B antigens including high mannose and branched polylactosamine. Thus, lectins identified subtle surface ABO blood group glycoprotein density variations not detected by routine serological methods. Transfusion services observe alterations in ABH expression during malignancy, and ABO incompatible solid organ transplantation is not without risk of rejection. The presented methods may identify subtle but clinically significant ABO blood group differences for transfusion and transplantation.

RGS10 and RGS18 differentially limit platelet activation, promote platelet production, and prolong platelet survival.

G protein-coupled receptors are critical mediators of platelet activation whose signaling can be modulated by members of the regulator of G protein signaling (RGS) family. The 2 most abundant RGS proteins in human and mouse platelets are RGS10 and RGS18. While each has been studied individually, critical questions remain about the overall impact of this mode of regulation in platelets. Here, we report that mice missing both proteins show reduced platelet survival and a 40% decrease in platelet count that can be partially reversed with aspirin and a P2Y12 antagonist. Their platelets have increased basal (TREM)-like transcript-1 expression, a leftward shift in the dose/response for a thrombin receptor-activating peptide, an increased maximum response to adenosine 5'-diphosphate and TxA2, and a greatly exaggerated response to penetrating injuries in vivo. Neither of the individual knockouts displays this constellation of findings. RGS10-/- platelets have an enhanced response to agonists in vitro, but platelet count and survival are normal. RGS18-/- mice have a 15% reduction in platelet count that is not affected by antiplatelet agents, nearly normal responses to platelet agonists, and normal platelet survival. Megakaryocyte number and ploidy are normal in all 3 mouse lines, but platelet recovery from severe acute thrombocytopenia is slower in RGS18-/- and RGS10-/-18-/- mice. Collectively, these results show that RGS10 and RGS18 have complementary roles in platelets. Removing both at the same time discloses the extent to which this regulatory mechanism normally controls platelet reactivity in vivo, modulates the hemostatic response to injury, promotes platelet production, and prolongs platelet survival.

ß4GALT1 controls ß1 integrin function to govern thrombopoiesis and hematopoietic stem cell homeostasis.

Glycosylation is critical to megakaryocyte (MK) and thrombopoiesis in the context of gene mutations that affect sialylation and galactosylation. Here, we identify the conserved B4galt1 gene as a critical regulator of thrombopoiesis in MKs. ß4GalT1 deficiency increases the number of fully differentiated MKs. However, the resulting lack of glycosylation enhances ß1 integrin signaling leading to dysplastic MKs with severely impaired demarcation system formation and thrombopoiesis. Platelets lacking ß4GalT1 adhere avidly to ß1 integrin ligands laminin, fibronectin, and collagen, while other platelet functions are normal. Impaired thrombopoiesis leads to increased plasma thrombopoietin (TPO) levels and perturbed hematopoietic stem cells (HSCs). Remarkably, ß1 integrin deletion, specifically in MKs, restores thrombopoiesis. TPO and CXCL12 regulate ß4GalT1 in the MK lineage. Thus, our findings establish a non-redundant role for ß4GalT1 in the regulation of ß1 integrin function and signaling during thrombopoiesis. Defective thrombopoiesis and lack of ß4GalT1 further affect HSC homeostasis.

B cells suppress medullary granulopoiesis by an extracellular glycosylation-dependent mechanism.

The immune response relies on the integration of cell-intrinsic processes with cell-extrinsic cues. During infection, B cells vacate the marrow during emergency granulopoiesis but return upon restoration of homeostasis. Here we report a novel glycosylation-mediated crosstalk between marrow B cells and hematopoietic progenitors. Human B cells secrete active ST6GAL1 sialyltransferase that remodels progenitor cell surface glycans to suppress granulopoiesis. In mouse models, ST6GAL1 from B cells alters the sialylation profile of bone marrow populations, and mature IgD+ B cells were enriched in sialylated bone marrow niches. In clinical multiple myeloma, ST6GAL1 abundance in the multiple myeloma cells negatively correlated with neutrophil abundance. These observations highlight not only the ability of medullary B cells to influence blood cell production, but also the disruption to normal granulopoiesis by excessive ST6GAL1 in malignancy.

Refrigeration-Induced Binding of von Willebrand Factor Facilitates Fast Clearance of Refrigerated Platelets.

OBJECTIVE: Apheresis platelets for transfusion treatment are currently stored at room temperature because after refrigeration platelets are rapidly cleared on transfusion. In this study, the role of von Willebrand factor (VWF) in the clearance of refrigerated platelets is addressed. APPROACH AND RESULTS: Human and murine platelets were refrigerated in gas-permeable bags at 4°C for 24 hours. VWF binding, platelet signaling events, and platelet post-transfusion recovery and survival were measured. After refrigeration, the binding of plasma VWF to platelets was drastically increased, confirming earlier studies. The binding was blocked by peptide OS1 that bound specifically to platelet glycoprotein (GP)Ibα and was absent in VWF-/- plasma. Although surface expression of GPIbα was reduced after refrigeration, refrigeration-induced VWF binding under physiological shear induced unfolding of the GPIbα mechanosensory domain on the platelet, as evidenced by increased exposure of a linear epitope therein. Refrigeration and shear treatment also induced small elevation of intracellular Ca2+, phosphatidylserine exposure, and desialylation of platelets, which were absent in VWF-/- platelets or inhibited by OS1, which is a monomeric 11-residue peptide (CTERMALHNLC). Furthermore, refrigerated VWF-/- platelets displayed increased post-transfusion recovery and survival than wild-type ones. Similarly, adding OS1 to transgenic murine platelets expressing only human GPIbα during refrigeration improved their post-transfusion recovery and survival. CONCLUSIONS: Refrigeration-induced binding of VWF to platelets facilitates their rapid clearance by inducing GPIbα-mediated signaling. Our results suggest that inhibition of the VWF-GPIbα interaction may be a potential strategy to enable refrigeration of platelets for transfusion treatment.

Circulating blood and platelets supply glycosyltransferases that enable extrinsic extracellular glycosylation.

Glycosyltransferases, usually residing within the intracellular secretory apparatus, also circulate in the blood. Many of these blood-borne glycosyltransferases are associated with pathological states, including malignancies and inflammatory conditions. Despite the potential for dynamic modifications of glycans on distal cell surfaces and in the extracellular milieu, the glycan-modifying activities present in systemic circulation have not been systematically examined. Here, we describe an evaluation of blood-borne sialyl-, galactosyl- and fucosyltransferase activities that act upon the four common terminal glycan precursor motifs, GlcNAc monomer, Gal(ß3)GlcNAc, Gal(ß4)GlcNAc and Gal(ß3)GalNAc, to produce more complex glycan structures. Data from radioisotope assays and detailed product analysis by sequential tandem mass spectrometry show that blood has the capacity to generate many of the well-recognized and important glycan motifs, including the Lewis, sialyl-Lewis, H- and Sialyl-T antigens. While many of these glycosyltransferases are freely circulating in the plasma, human and mouse platelets are important carriers for others, including ST3Gal-1 and ß4GalT. Platelets compartmentalize glycosyltransferases and release them upon activation. Human platelets are also carriers for large amounts of ST6Gal-1 and the α3-sialyl to Gal(ß4)GlcNAc sialyltransferases, both of which are conspicuously absent in mouse platelets. This study highlights the capability of circulatory glycosyltransferases, which are dynamically controlled by platelet activation, to remodel cell surface glycans and alter cell behavior.

Platelet clearance by the hepatic Ashwell-Morrell receptor: mechanisms and biological significance.

The daily production of billions of platelets must be regulated to avoid spontaneous bleeding or arterial occlusion and organ damage. Complex mechanisms control platelet production and clearance in physiological and pathological conditions. This review will focus on the mechanisms of platelet senescence with specific emphasis on the role of post-translational modifications in platelet life-span and thrombopoietin production downstream of the hepatic Ashwell-Morrell receptor.

Novel mechanisms of platelet clearance and thrombopoietin regulation.

PURPOSE OF REVIEW: The human body produces and removes 10 platelets daily to maintain a normal steady-state platelet count. Platelet production must be tightly regulated to avoid spontaneous bleeding or arterial occlusion and organ damage. Multifaceted and complex mechanisms control platelet removal and production in physiological and pathological conditions. This review will focus on different mechanisms of platelet clearance, with focus on the biological significance of platelet glycans. RECENT FINDINGS: The Ashwell-Morrell receptor (AMR) recognizes senescent, desialylated platelets under steady state conditions. Desialylated platelets and the AMR are the physiological ligand-receptor pair regulating hepatic thrombopoietin (TPO) mRNA production, resolving the longstanding mystery of steady state TPO regulation. The AMR-mediated removal of desialylated platelets regulates TPO synthesis in the liver by recruiting JAK2 and STAT3 to increase thrombopoiesis. SUMMARY: Inhibition of TPO production downstream of the hepatic AMR-JAK2 signaling cascade could additionally contribute to the thrombocytopenia associated with JAK1/2 treatment, which is clinically used in myeloproliferative neoplasms.

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

Dynamins are highly conserved large GTPases (enzymes that hydrolyze guanosine triphosphate) involved in endocytosis and vesicle transport, and mutations in the ubiquitous and housekeeping dynamin 2 (DNM2) have been associated with thrombocytopenia in humans. To determine the role of DNM2 in thrombopoiesis, we generated Dnm2(fl/fl) Pf4-Cre mice specifically lacking DNM2 in the megakaryocyte (MK) lineage. Dnm2(fl/fl) Pf4-Cre mice had severe macrothrombocytopenia with moderately accelerated platelet clearance. Dnm2-null bone marrow MKs had altered demarcation membrane system formation in vivo due to defective endocytic pathway, and fetal liver-derived Dnm2-null MKs formed proplatelets poorly in vitro, showing that DNM2-dependent endocytosis plays a major role in MK membrane formation and thrombopoiesis. Endocytosis of the thrombopoietin receptor Mpl was impaired in Dnm2-null platelets, causing constitutive phosphorylation of the tyrosine kinase JAK2 and elevated circulating thrombopoietin levels. MK-specific DNM2 deletion severely disrupted bone marrow homeostasis, as reflected by marked expansion of hematopoietic stem and progenitor cells, MK hyperplasia, myelofibrosis, and consequent extramedullary hematopoiesis and splenomegaly. Taken together, our data demonstrate that unrestrained MK growth and proliferation results in rapid myelofibrosis and establishes a previously unrecognized role for DNM2-dependent endocytosis in megakaryopoiesis, thrombopoiesis, and bone marrow homeostasis.

The Ashwell-Morell receptor regulates hepatic thrombopoietin production via JAK2-STAT3 signaling.

The hepatic Ashwell-Morell receptor (AMR) can bind and remove desialylated platelets. Here we demonstrate that platelets become desialylated as they circulate and age in blood. Binding of desialylated platelets to the AMR induces hepatic expression of thrombopoietin (TPO) mRNA and protein, thereby regulating platelet production. Endocytic AMR controls TPO expression through Janus kinase 2 (JAK2) and the acute phase response signal transducer and activator of transcription 3 (STAT3) in vivo and in vitro. Recognition of this newly identified physiological feedback mechanism illuminates the pathophysiology of platelet diseases, such as essential thrombocythemia and immune thrombocytopenia, and contributes to an understanding of the mechanisms of thrombocytopenia observed with JAK1/2 inhibition.