Activin Receptor Research Articles

Therapeutic Cell Repopulation of the Liver: From Fetal Rat Cells to Synthetic Human Tissues.

Progenitor cells isolated from the fetal liver can provide a unique cell source to generate new healthy tissue mass. Almost 20 years ago, it was demonstrated that rat fetal liver cells repopulate the normal host liver environment via a mechanism akin to cell competition. Activin A, which is produced by hepatocytes, was identified as an important player during cell competition. Because of reduced activin receptor expression, highly proliferative fetal liver stem/progenitor cells are resistant to activin A and therefore exhibit a growth advantage compared to hepatocytes. As a result, transplanted fetal liver cells are capable of repopulating normal livers. Important for cell-based therapies, hepatic stem/progenitor cells containing repopulation potential can be separated from fetal hematopoietic cells using the cell surface marker δ-like 1 (Dlk-1). In livers with advanced fibrosis, fetal epithelial stem/progenitor cells differentiate into functional hepatic cells and out-compete injured endogenous hepatocytes, which cause anti-fibrotic effects. Although fetal liver cells efficiently repopulate the liver, they will likely not be used for human cell transplantation. Thus, utilizing the underlying mechanism of repopulation and developed methods to produce similar growth-advantaged cells in vitro, e.g., human induced pluripotent stem cells (iPSCs), this approach has great potential for developing novel cell-based therapies in patients with liver disease. The present review gives a brief overview of the classic cell transplantation models and various cell sources studied as donor cell candidates. The advantages of fetal liver-derived stem/progenitor cells are discussed, as well as the mechanism of liver repopulation. Moreover, this article reviews the potential of in vitro developed synthetic human fetal livers from iPSCs and their therapeutic benefits.

Follistatin Forms a Stable Complex With Inhibin A That Does Not Interfere With Activin A Antagonism.

Inhibins are transforming growth factor-β family heterodimers that suppress follicle-stimulating hormone (FSH) secretion by antagonizing activin class ligands. Inhibins share a common β chain with activin ligands. Follistatin is another activin antagonist, known to bind the common β chain of both activins and inhibins. In this study, we characterized the antagonist-antagonist complex of inhibin A and follistatin to determine if their interaction impacted activin A antagonism. We isolated the inhibin A:follistatin 288 complex, showing that it forms in a 1:1 stoichiometric ratio, different from previously reported homodimeric ligand:follistatin complexes, which bind in a 1:2 ratio. Small angle X-ray scattering coupled with modeling provided a low-resolution structure of inhibin A in complex with follistatin 288. Inhibin binds follistatin via the shared activin β chain, leaving the α chain free and flexible. The inhibin A:follistatin 288 complex was also shown to bind heparin with lower affinity than follistatin 288 alone or in complex with activin A. Characterizing the inhibin A:follistatin 288 complex in an activin-responsive luciferase assay and by surface plasmon resonance indicated that the inhibitor complex readily dissociated upon binding type II receptor activin receptor type IIb, allowing both antagonists to inhibit activin signaling. Additionally, injection of the complex in ovariectomized female mice did not alter inhibin A suppression of FSH. Taken together, this study shows that while follistatin binds to inhibin A with a substochiometric ratio relative to the activin homodimer, the complex can dissociate readily, allowing both proteins to effectively antagonize activin signaling.

Thrombosis in hereditary hemorrhagic telangiectasia

Thrombosis in hereditary hemorrhagic telangiectasia

A Potential Negative Regulatory Function of Myostatin in the Growth of the Pacific Abalone, Haliotis discus hannai.

Myostatin, also known as GDF8, is a member of the transforming growth factor-β (TGF-β) superfamily. In vertebrates, myostatin negatively regulates the growth of skeletal muscle. In invertebrates, it has been reported to be closely related to animal growth. However, knowledge concerning the molecular mechanisms involved in the myostatin regulation of molluscan growth is limited. In this study, we found that the hdh-myostatin open reading frame (ORF) comprised 1470 base pairs that encoded 489 amino acids and contained structural characteristics typical of the TGF-β superfamily, including a C-terminal signal peptide, a propeptide domain, and TGF-β region. Gene expression analysis revealed that hdh-myostatin mRNA was widely expressed at different levels in all of the examined tissues of Haliotis discus hannai. Nine single nucleotide polymorphisms (SNPs) were associated with the growth traits. RNA interference (RNAi) against hdh-myostatin mRNA significantly downregulated hdh-myostatin at days 1, 15, and 30 post injection, and the pattern was correlated with downregulation of the genes TGF-β receptor type-I (hdh-TβR I), activin receptor type-IIB (hdh-ActR IIB), and mothers against decapentaplegic 3 (hdh-Smad3). After one month of the RNAi experiment, the shell lengths and total weights increased in the abalone, Haliotis discus hannai. The results of qRT-PCR showed that the hdh-myostatin mRNA level was higher in the slow-growing group than in the fast-growing group. These results suggest that hdh-myostatin is involved in the regulation of growth, and that these SNPs would be informative for further studies on selective breeding in abalone.

Protein-to-protein interaction of genes responsible for the economic trait of Madura Cattle: an in silico analysis

Our previous study found five genes consist of Insulin-like growth factor-1 (IGF-1), Growth hormone (GH1), Growth hormone receptor (GHR), Myostatin (MSTN), and Leptin (LEP) are responsible for the economic traits of Madura Cattle. This paper aimed to identify the protein-to-protein interaction of genes responsible for the economic traits of Madura Cattle. In silico study was done using STRING v.11.5 with Bos taurus IGF1, GH1, GHR, MSTN, and LEP as input analysis. Results show 24 biological processes, 3 molecular functions, and 6 KEGG pathways as potential protein interactions between genes. All genes are involved in six biological processes i.e. response to the hormone, response to oxygen-containing compound, cell surface receptor signalling pathway, regulation of signal transduction, positive regulation of the cellular metabolic process, and positive regulation of the macromolecule metabolic process. Five genes were predicted as functional partner genes, namely Leptin receptor (LEPR), Tyrosine-protein kinase receptor (INSR), Activin receptor type-2b precursor (ACVR2B), Insulin-like growth factor 1 receptor (IGF1R), and Insulin-like growth factor-binding protein 3 (IGFBP3). Based on the results, we can demonstrate the protein-to-protein interaction of IGF-1, GH, GHR, MSTN, and LEP genes which are potentially related to each other. Further experimental study is required to validate the interaction.

Activin receptor ALK4 promotes adipose tissue hyperplasia by suppressing differentiation of adipocyte precursors

Adipocyte hyperplasia and hypertrophy are the two main processes contributing to adipose tissue expansion, yet the mechanisms that regulate and balance their involvement in obesity are incompletely understood. Activin B/GDF-3 receptor ALK7 is expressed in mature adipocytes and promotes adipocyte hypertrophy upon nutrient overload by suppressing adrenergic signaling and lipolysis. In contrast, the role of ALK4, the canonical pan-activin receptor, in adipose tissue is unknown. Here, we report that, unlike ALK7, ALK4 is preferentially expressed in adipocyte precursors, where it suppresses differentiation, allowing proliferation and adipose tissue expansion. ALK4 expression in adipose tissue increases upon nutrient overload and positively correlates with fat depot mass and body weight, suggesting a role in adipose tissue hyperplasia during obesity. Mechanistically, ALK4 signaling suppresses expression of CEBPα and PPARγ, two master regulators of adipocyte differentiation. Conversely, ALK4 deletion enhances CEBPα/PPARγ expression and induces premature adipocyte differentiation, which can be rescued by CEBPα knockdown. These results clarify the function of ALK4 in adipose tissue and highlight the contrasting roles of the two activin receptors in the regulation of adipocyte hyperplasia and hypertrophy during obesity.

ALK2 Inhibition and a Modified Activin Receptor Type Iia Ligand Trap Cotherapy Maximized Hematologic Improvements in a Mouse Model of Anemia of Inflammation

Introduction: Anemia of inflammation (AI) is commonly seen in patients with chronic diseases, including chronic kidney disease (CKD). Current therapeutic options to mitigate anemia in AI include treating the underlying inflammatory disease, oral or intravenous iron supplementation, erythropoietin, and blood transfusions. The persistent release of inflammatory cytokines in AI not only induces the iron regulator hepcidin to inhibit iron absorption and recycling, but also directly impacts the marrow to suppress erythropoiesis, further worsening the ineffective, iron-restricted erythropoiesis. Therefore, targeting both iron and erythropoiesis could comprehensively address anemia in AI. We previously showed that ALK2 inhibition was sufficient to reduce serum hepcidin and improve hemoglobin in a CKD mouse model. We also demonstrated that a modified activin receptor IIA ligand trap RKER-050 increased hemoglobin and red blood cell production by promoting erythroid differentiation in mice. Here, we explored the potential of additive improvements in hematologic parameters that could be achieved with a combination of hepcidin suppression by ALK2 inhibition and increased hematopoiesis with RKER-050 in a mouse model of AI. Methods: C57BL/6 mice were fed a control or 0.2% adenine with 40 ppm iron diet (Ade 40) for 5 weeks to induce CKD. After confirming anemia, CKD mice were subcutaneously treated with 3 mg/kg isotype control or KTI-m216, a neutralizing antibody against ALK2, twice a week for 4 weeks while continuing the Ade 40 diet or switching to a 0.2% adenine with 3 ppm low iron diet (Ade 3). Combination therapy was evaluated in CKD mice on Ade 40 diet receiving biweekly isotype or KTI-m216 and also received either weekly PBS or 7.5 mg/kg RKER-050 intraperitoneally for 4 weeks. Results: CKD mice developed impaired kidney function and characteristics of AI, including increased serum IL-6 and hepcidin, hypoferremia, tissue iron retention, and anemia compared to control mice. Serum hepcidin was reduced >95% in KTI-m216 treated mice on both Ade diets compared to mice receiving isotype control (CKD control). Transferrin saturation (TSAT) increased 65% in KTI-m216 treated mice on Ade 40 diet, with a similar trend in mice on Ade 3 diet. When compared to CKD control of the same diet, KTI-m216 treated mice had similar hematologic improvements: hemoglobin (+2.2 g/dL), hematocrit (+5.4%), red blood cells (+1.1 M/μl), and reticulocyte hemoglobin (CHr, +1.8 pg). These data support that increased circulating iron by KTI-m216 mediated hepcidin suppression can increase erythropoiesis in CKD mice. Data also suggest that dietary iron contribution is negligible in supporting erythropoiesis in this context. With combination therapy, mice receiving KTI-m216 alone (KTI-m216 and PBS) and a combined KTI-m216 and RKER-050 therapy had a similar reduction in serum hepcidin (>91%) and a similar increase in TSAT (>1.7-fold), compared to mice receiving vehicle (isotype and PBS) controls. Mice treated with KTI-m216 alone increased hemoglobin to 11.4 g/dL compared to vehicle treated mice with hemoglobin of 8.0 g/dL. Treatment with a combined KTI-m216 and RKER-050 therapy showed a further increase in hemoglobin to 13.5 g/dL. Additionally, red blood cell production was 6.4 M/μl in mice given vehicle controls and increased to 8.3 M/μl in KTI-m216 alone and 9.8 M/μl in combined KTI-m216 and RKER-050; hematocrit was 23.4% in vehicle control, 32.5% in KTI-m216 alone and 38.7% in combined KTI-m216 and RKER-050. Although hematocrit was not fully rescued, hemoglobin and red blood cells in mice receiving a combined KTI-m216 and RKER-050 therapy were not different from the non-CKD controls. Conclusion: These data illustrate that inhibition of ALK2 with KTI-m216 could potentially release sufficient iron from the recycling pathway to ameliorate anemia, but not restore RBC parameters to normal levels. Combined treatment with activin receptor ligand trap, RKER-050 and KTI-m216 normalized the RBC production in this preclinical model. These data support that increasing erythropoiesis with RKER-050 combined with iron mobilization with KTI-m216 has the potential to achieve the maximal amelioration of anemia in AI.

Combination of a TGF-β Ligand Trap (RAP-GRL) and TMPRSS6-ASO Is Superior for Correcting β-Thalassemia

The hallmarks of β-thalassemia (BT) include ineffective erythropoiesis (IE) and iron overload (IO). Our studies have pointed to iron restriction (IR) to improve both anemia and IO in BT. One such IR therapy targets the matriptase-2 (TMPRSS6) gene using antisense oligonucleotides (ASO). As shown by Guo et al JCI 2013, treatment of Hbbth3/+(th3/+) mice (a BT mouse model for BT-intermedia) with TMPRSS6-ASO (T-ASO) lengthens RBC lifespan, ameliorates splenomegaly and improves anemia. A novel therapeutic approach to improve anemia targets the TGF-β pathway to promote red blood cell (RBC) production. Luspatercept, a Transforming Growth Factor (TGF)-β ligand-trap that is modeled after the extracellular domain of activin receptor (ACVR)-2B, gained FDA approval in 2019 to treat transfusion dependent BT patients (Cappellini and Taher Blood Adv, 2021). In mouse models of BT, its murine analog (RAP-536) was suggested to promote erythropoietin (EPO) independent maturation of late-stage erythroid cells, resulting in increased RBC parameters in a dose-dependent manner (Surgani et al Nat. Med., 2014). In clinical trials, the drug has been shown to improve anemia and reduce the need for blood transfusion in BT. However, despite its success, it does not improve anemia in all patients. Therefore, we hypothesized that a combinatorial approach could be more effective, and potentially beneficial to a greater number of patients. In this work, we explored a combination approach using an agent analogous to murine Luspatercept (RAP-GRL) and iron restriction (IR) using T-ASO, with the goals of targeting distinct morbidities associated with BT and further improve anemia. To test our construct, RAP-GRL was produced in a mammalian cell line and purified. Wild-type (WT) or th3/+ mice were subcutaneously (s.c.) injected with 10mg/kg of RAP-GRL and monitored by complete blood counts. Treatment with purified RAP-GRL increased RBC parameters in both WT and th3/+ mice (n=3-9, 2-4-month-old females and males). In combination therapy experiments, we injected s.c. with 10mg/kg of RAP-GRL and/or 5mg/kg T-ASO via intraperitoneal (i.p.) injection in th3/+ mice. At the experimental end point (6 weeks), RBC parameters were increased in all treatment groups, except in controls treated with PBS+Scramble-ASO (Fig.1a-b). Flow cytometry analysis using CD71, TER119, and CD44 antibodies showed improvements of ineffective erythropoiesis in the bone marrow (BM) and spleen (SPL) of all treatment groups compared to control PBS+Scramble-ASO treated animals. In addition, our data suggest that RAP-GRL does not directly modulate systemic iron metabolism (Fig.1c). Additionally, ROS levels and splenomegaly were also greatly reduced in T-ASO and RAP-GRL+T-ASO groups. Serum assessment of T-ASO and RAP-GRL+T-ASO treated animals showed decreased levels of transferrin saturations with a simultaneous increase in hepcidin levels, but not in the RAP-GRL treated groups. Erythroferrone (ERFE) levels were decreased in all T-ASO and RAP-GRL+T-ASO groups. However, compared to th3/+ T-ASO treated mice, EPO levels were increased in the RAP-GRL treated group (Fig.1d). Additionally, although EPO was relatively elevated in all RAP-GRL treated groups, only the RAP-GRL+T-ASO group had reduced ERFE. This result is in agreements with our findings of decreased early (ERFE-producing) erythroid progenitors in the BM and SPL of RAP-GRL+T-ASO treated mice. Interestingly, our data also highlight the superior ability of T-ASO in improving erythroid maturation, especially considering the reduced EPO requirement. Preliminary RBC metabolomic data shows differences in redox metabolites, coenzymes and metabolites involved in protein synthesis of both RAP-GRL and T-ASO singly treated groups with additive effects in the combination group, in line with the observed improved RBC quality and survival. Further analyses are under way. In conclusions, we observed that T-ASO is superior in reducing EPO requirement, improving iron metabolism and red cell differentiation, while RAP-GRL increases overall RBC production. Their combination maintains these positive endpoints, but also is also superior in improving RBC and Hb synthesis, with a significant improvement in the anemia. This study provides the first pre-clinical support for combining these two drugs to improve anemia as well as iron metabolism in BT patients. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Investigations into the Mechanisms and Clinical Implications of Modulation of Hepcidin Levels By Luspatercept in TD MDS and TD b-Thalassemia

Introduction: Multiple blood transfusions lead to significant iron overload in transfusion dependent (TD) thalassemia (TDT) and TD-MDS. This excess iron is deposited in various organs leading to end organ damage, morbidity, and mortality. Iron overload patients are treated with iron chelation therapy (ICT) owing to lack of effective physiological iron removal mechanisms. Furthermore, chronic transfusions lead to paradoxical changes in hepcidin, the master regulator of iron homeostasis. While hepcidin is upregulated under conditions of increased intracellular and extracellular iron concentrations through BMP6 and IL6 dependent signaling; it is downregulated during expanded erythropoiesis through Erythroferrone (ERFE) to increase iron availability for developing erythron. Increased levels of hepcidin sequesters iron within organs leading to inefficiency of iron chelation therapy in TD-MDS and TDT patients. Luspatercept is a modified activin receptor fusion protein (modified ActRIIB-Fc) that sequesters certain members of TGF-b superfamily ligands, thus enhancing late-stage erythroid-maturation. Luspatercept is approved for TDT and TD-MDS patient populations-based on significant reduction in transfusion burden in two independent phase 3 trails (BELIEVE and MEDALIST, respectively). Here, we evaluated modulation of hepcidin levels with luspatercept treatment and its clinical implications in anemias due to elevated levels of hepcidin including TD-MDS and TDT patients. Methods: Luspatercept was administered sub-cutaneously every three weeks. Hepcidin and ERFE levels were measured in serum or plasma samples from BELIEVE (NCT02604433) and MEDALIST (NCT02631070) trial patients using specific ELISA and or chemiluminescence assay kits at week 25 following treatment. The regulation of hepcidin by luspatercept, BMP6 and IL6 in HepG2 cell-line model was determined by ELISA (in conditioned media) and q-PCR at increasing doses and time points. Results: Serum hepcidin levels at baseline across both arms in MEDALIST and BELIEVE were 80.65 ng/ml and 43.1 ng/ml respectively. These were significantly higher than normal healthy individuals (range of 5-10 ng/ml; P < 0.001), reported previously (Ilkovska etal JIMB 2016). Patients treated with luspatercept in both TDT and TD-MDS showed significant reduction in serum levels of hepcidin by 49% and 53% respectively (P<0.001) at week 25 compared to baseline. Hepcidin levels in the placebo group in TD-MDS patients demonstrated an increase of 119% (P<0.001) and a decrease of 12% in TDT patients (P=0.004) at the same time point. In addition, ERFE protein levels in serum, were elevated in response to Luspatercept treatment in both TD-MDS (30%) and TDT (51%) (P<0.001) in comparison to the placebo arms of decrease of 28% in TD-MDS and an increase of 2% in TDT. In HepG2 cells, treatment with recombinant BMP6 and IL6 increased hepcidin protein expression in a dose dependent manner. Luspatercept treatment inhibited this increase in hepcidin expression mediated by BMP6 and IL6. Time course analyses (2, 6, 24 and 72 hours), revealed that luspatercept inhibited hepcidin transcription and secretion levels starting at 24 hours. Conclusions: Luspatercept treatment reduced transfusion burden and iron loading rate due to improved erythropoiesis and release of ERFE by maturing erythroblasts consequently leading to reductions in elevated hepcidin levels in both TD-MDS and TDT patients. Our data provides a mechanistic rationale for downregulation of hepcidin levels with luspatercept treatment. Interestingly, cell-based assay data suggest that luspatercept directly modulates hepcidin transcription suggesting that in diseased conditions with abnormally higher levels of hepcidin and TGFβ family ligands, luspatercept might also modulate hepcidin transcription by sequestering TGFβ family ligands implicated in hepcidin synthesis. We postulate that decreased hepcidin levels results in release of excess iron into peripheral blood, making it available for effective chelation. These observations implicate sequencing of luspatercept, and ICT for effective management of iron overload in these TD patients. Furthermore, the data provide a mechanistic rationale for evaluating luspatercept in hepcidin dependent anemia of inflammation and iron refractory iron deficiency anemia (IRIDA) disease indications.

Rker-050, a Novel Activin Receptor Type II Ligand Trap, Rescued Anemia and Reduced Bone Loss in a Mouse Model of Myelodysplastic Syndromes

Myelodysplastic syndromes (MDS) are a group of hematopoietic stem cell diseases characterized by ineffective hematopoiesis, peripheral blood cytopenias and dysplasias resulting from molecular defects and dysregulated signaling that govern the regulation of hematopoietic stem and progenitor cells (HSPCs). Crosstalk between cells in the osteo-hematopoietic (O-H) niche are necessary for the maintenance and self-renewal of HSPCs. Additionally, MDS patients commonly present with osteopenia and osteoporosis further demonstrating the critical interplay between the O-H cells in the bone marrow (BM). Alterations in the interaction between bone precursors and HSPCs within the BM microenvironment have been linked to the pathophysiology of MDS. Transforming growth factor-beta (TGF-β) superfamily signaling is instrumental in regulating both normal hematopoiesis and bone homeostasis. KER-050 is a modified investigational activin receptor type II ligand trap designed to bind and inhibit select TGF-β superfamily ligands, including activin A, activin B, GDF8 and GDF11, to promote hematopoiesis and bone growth. Preclinical studies have demonstrated that treatment with a research form of KER-050 (RKER-050) increased bone mass and multi-lineage hematopoiesis in healthy mice. To further investigate the effect of RKER-050 on hematopoietic and bone systems within the MDS disease setting, the NUP98/HOX13 MDS mouse model was utilized. Five-month-old male MDS mice were treated with vehicle (VEH, n=10), or RKER-050 (050, 7.5mg/kg, n=9) once weekly for 6 weeks along with a wildtype vehicle (WT, n=7) control group. To assess changes in hematological parameters, CBCs were measured, bone marrow (BM) and splenocytes were characterized via flow cytometry. To investigate alterations of bone microarchitecture, femurs were assessed using microCT imaging. MDS-VEH mice showed a reduction in red blood cells (RBC; 30%), hemoglobin (HGB; 19%), hematocrit (HCT; 13%) and reticulocytes (Retic; 16%) compared to WT; all hallmarks of human MDS. Relative to MDS-VEH, MDS-050 showed increased RBC, HGB, HCT and Retic by 25%, 13%, 10% and 39%, respectively. The erythroid Ter119 positive (Ter119+) cell population was slightly reduced in BM and spleen of MDS-VEH mice compared to WT. The BM Ter119+ cells were unchanged in the MDS-050 cohort relative to MDS-VEH; however, the spleen Ter119+ cells were increased similar to the levels in WT mice. Specific erythroid precursor populations were increased in BM and spleen of MDS-VEH mice versus WT, consistent with the ineffective erythropoiesis in this model. Interestingly, erythroid precursor populations were similar between MDS-VEH and MDS-050, yet MDS-050 exhibited recovered RBC production, suggesting an increase in maturation of erythroid precursors with RKER-050 treatment. Given the importance of the O-H niche in regulating hematopoiesis along with osteoporosis and osteopenia being comorbidities of MDS, bone microarchitecture was also analyzed. In the distal femoral trabecular bone, relative to WT mice, MDS-VEH mice had significant increases in trabecular (Tb) separation (33%) and decreases in Tb number (28%), changes that are consistent with reduced bone strength. Bone volume (34%) and bone volume fraction (26%) decreased but were not statistically significant. MDS-050 treated mice had a significant increase in bone volume (71%), higher bone volume fraction (88%), increased Tb number (44%) and decreased Tb spacing (20%) compared to the MDS-VEH, supporting RKER-050's role in improving bone mass. Taken together, these results suggest that RKER-050 can mitigate anemia and bone loss in an MDS mouse model, potentially by rebalancing hematopoiesis and bone turnover. In particular, the ability of RKER-050 to improve bone microarchitecture within the trabecular region, where the O-H niche resides, could be an important step in reestablishing healthy blood cell production. Furthermore, exploratory PD data from an ongoing phase 2 clinical trial demonstrate improved blood parameters with KER-050 treatment, including platelet and RBC in MDS patients (Tan et al. 2022 EHA #P776), and changes in bone biomarkers are currently being evaluated in the study. KER-050, thus, represents a potentially promising approach for patients with MDS and other hematological diseases, including myelofibrosis, where ineffective hematopoiesis and defects in bone homeostasis occur.

Effects of Ker-050 on Iron Metabolism: Exploratory Analyses from an Ongoing Phase 2 Study in Patients with Myelodysplastic Syndromes

Background: Iron overload (IO) is an important clinical consequence of transfusion-dependent myelodysplastic syndromes (MDS). For patients with high transfusion burden (HTB), limited treatment options are available to effectively treat anemia and reduce transfusion dependence. Transfusion dependence is an independent predictor of poor prognosis in MDS. It is associated with IO and elevated ferritin, a marker of intracellular iron storage that also predicts poor prognosis in MDS. When adjusting for transfusion burden, each 500 µg/L increase in ferritin above 1000 µg/L is associated with a 30% greater risk of death (Malcovati et al. 2006). IO also disrupts osteoprogenitors and erythroid precursors within the osteohematopoietic niche, leading to abnormal bone turnover and ineffective erythropoiesis (Bulycheva 2015). While iron chelators have shown some efficacy in managing IO, they introduce toxicities and have little effect on transfusion burden. Unmet need remains for treatments that effectively address anemia, IO, and reduce transfusion dependence in patients with MDS, especially those with HTB. KER-050 is designed to alter signaling of TGF-β superfamily ligands, restoring balance to the osteohematopoietic niche, and promoting differentiation of erythroid and megakaryocytic precursors. It is a modified activin receptor type IIA ligand trap that inhibits TGF-β superfamily ligands, including activins A and B and growth and differentiation factors (GDFs) 8 and 11. To date, clinical data indicate that KER-050 induces sustained increases in reticulocytes and hemoglobin. Preliminary findings also show potential for KER-050 to induce erythroid response and reduce transfusion burden in patients with MDS, including those with HTB (Tan et al., 2022). KER-050 may also impact iron metabolism and this presentation explores the effects of KER-050 on indices of IO. Methods: This Phase 2 study is evaluating KER-050 in participants with very low- to intermediate-risk MDS. Data presented are from Part 1 Dose Escalation which included 16 weeks of KER-050 treatment across 5 cohorts (0.75 to 5 mg/kg). Iron chelator therapy was permitted at the discretion of the investigator provided patients had been on a stable dose for 8 weeks prior to C1D1. Markers of IO and hematopoiesis were assessed as exploratory endpoints over the 16-week treatment period. Analyses presented evaluate the effect of KER-050 on these markers. Baseline markers of bone turnover were also assessed. Results: A total of 31 patients were enrolled in Part 1 Dose Escalation; 26 were transfusion dependent, 18 with HTB (≥ 4 RBC units in 8 weeks prior to day 1). Seven HTB patients (38.9%) received iron chelator therapy. Baseline differences were noted in measures of iron status, erythropoiesis and bone turnover for non-transfused (NT), low transfusion burden (LTB), and HTB patients (Table 1). HTB patients had the highest baseline serum iron and ferritin levels. Preliminary findings indicate that KER-050 treatment was associated with decreases in serum ferritin for patients both with and without iron chelator therapy. Patients achieving transfusion independence (TI) experienced a 39.5% reduction (mean change: -619.4 ng/ml, range: -2118.4 to +243 ng/mL) in serum ferritin at Week 12 that was generally sustained at Week 16 and 20 (Figure 1). Similarly, over the same time period, serum iron and transferrin saturation (TSAT) decreased by 4.03 µmol/L (8.38%) and 5.3%, respectively, in patients achieving TI. Soluble transferrin receptor (sTfR) increased by 32.2% and remained elevated at subsequent visits. Summary: Preliminary data from the 16-week Dose Escalation in this ongoing phase 2 study reveal differences in baseline iron biomarkers indicative of increasing iron overload with increasing transfusion burden. While nearly half of HTB patients received iron chelator therapy, this group had markedly elevated ferritin compared to NT and LTB patients at baseline. KER-050 treatment was associated with reductions in serum ferritin, irrespective of the use of iron chelation. Patients achieving TI experienced decreases in ferritin, iron and TSAT concurrent with increases in sTfR that were generally maintained through follow-up. The data suggest that, in addition to improving hematopoiesis and transfusion burden in MDS patients, KER-050 may reduce iron overload, a serious clinical complication impacting the survival of these patients. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

First Single-Center Experience with Luspatercept Therapy in Low-Risk Myelodysplastic Syndrome (LR-MDS) Patients with Transfusion Dependence Refractory to Erythropoietin Therapy

Introduction: In the treatment of low-risk myelodysplastic syndromes (MDS) patients, anemia is the predominant problem. A recent breakthrough in the therapy of anemia in low-risk MDS patients are drugs involved in TGFβ pathway blockade. Luspatercept is a recombinant fusion protein derived from human activin receptor type IIB. It binds TGFβ superfamily ligands and inhibits the SMAD2/3 pathway leading to potentiation of red cell line differentiation and proliferation. Luspatercept was approved last year by the FDA and EMEA in the treatment of anemic low-risk MDS patients with ring sideroblasts (MDS-RS-MLD, MDS-RS-SLD) and/or SF3B1 mutated patients who are not eligible for erythropoiesis-stimulating agents (ESA) or for patients who failed on this therapy. Luspatercept showed high clinical activity for the treatment of anemia in the phase 2 trial (PACE-MDS) and subsequently in the phase 3 trial (MEDALIST). It has led to transfusion independence (TI) in about 40 % of patients. Methods: From January 2021 to July 2022, 27 patients were treated with luspatercept at the hematology department of Charles University Hospital in Prague, Czech Republic. The median age of the patients was 77 years (range 57-89) with a male to female ratio 17:10. WHO 2016 diagnosis were: MDS-RS-MLD (N = 23) and MDS/MPN-RS-T (N = 4). Median of IPSS-R was 2.5 (range 1-4). Median of IPSS-R cytogenetic score scaled from 0-2 with the median of 1. Median follow-up was 7.8 months (range 1-38). Luspatercept was administered in subcutaneous injections once every 3 weeks with an initial dose of 1 mg/kg. Dose escalation was done according to the level of response to 1.33 and 1.75 mg/kg. The median number of cycles administered was 7 (2-17). Transfusion dependency before luspatercept initiation ranged from 2 transfusion units (TU)/8 weeks to 12 TU/8 weeks, 11 patients had low transfusion burden (LTB), 16 high transfusion burden (HTB). Baseline erythropoietin (EPO) level was 374 IU/l in average (range 150-770) and ferritin level was 1307 μg/l in average (range 234-3832). 9 patients received chelation therapy. 22 patients were tested for SF3B1 mutation with 21 positive results (95.5 %) using Sanger sequencing (N = 3) or NGS approach (N = 18). Isolated mutation of SF3B1 was present in 8 patients (44 %). The most frequent mutations besides SF3B1 were TET2 (N = 8; 44 %), ASXL1 (N = 3; 17 %), DNMT3A and RUNX1 (N = 2; 11 %). Average mutations count per one patient was 1.7 (range 0-5). We did not observe any TP53 mutation. Results: There were 24 evaluable patients in terms of response. Thus far 11 (45 %) patients have responded by achieving TI, 5 (20 %) patients have had a reduction in transfusion dependence (HI, according to IWG criteria 2006), 8 (33 %) patients have not responded. One patient progressed to the higher risk MDS, and one transformed to acute myeloid leukemia. Median duration of response was 7.4 months (range: 0.7-31.8). Median number of cycles to achieve response was 4. Eleven patients responded to the basic dose 1 mg/kg, 2 additional patients on the dose 1.33 mg/kg and 2 more on the dose 1.75 mg/kg. There were differences in response according to transfusion burden, in LTB group achieved TI 8 patients and HI 1 patient, in HTB group we observed TI in 3 patients and HI in 4 patients. In 9 patients, we added ESA to luspatercept therapy, as they act both at different levels of erythropoiesis, which led to the improvement of response in 5 cases. In general, patients with lower initial baseline EPO level were better responders. We did not observe decrease in ferritin level during therapy. There was no significant effect of mutation burden on patient's OS, although patients with single mutated SF3B1 achieved better ORR (75 %), than patients bearing mutations in multiple genes (ORR = 40 %). There were no adverse effects of Grade II or more. Conclusion: In conclusion, luspatercept is certainly the next major step in the treatment of anemia in MDS patients. The advantage of the drug is very good tolerance and easy administration. We observed better responses in patients bearing single mutation in SF3B1, patients with LTB and lower initial baseline EPO levels. The higher response rate in our follow-up may be influenced by the combination with ESA and rapid dose escalation. This work was supported by the following grants: grant AZV 2021-2024 NU21-08-00312, Grant AZV 2019-2023 NV19-08-00144, UNCE/MED/016

Modulation of TGF-β Superfamily Signaling to Treat Myelofibrosis and Mitigate JAK Inhibitor Toxicity: A Report on the Phase 2 Study of Ker-050 in Participants with Myelofibrosis

Background: Myelofibrosis (MF) is a disorder characterized by unrestrained proliferation of myeloid precursors and dysfunctional Janus kinase (JAK) signaling. Hyperplastic and dysplastic megakaryocyte expansion in MF releases inflammatory cytokines. These cytokines promote fibroblast proliferation, bone remodeling, and marrow fibrosis, disrupting the osteohematopoietic niche and leading to ineffective and extramedullary hematopoiesis (Vainchenker et al. 2017). Clinical manifestations include splenomegaly, anemia, and often disabling constitutional symptoms. JAK inhibitors, such as ruxolitinib, have been shown to improve spleen size and symptoms, but are associated with dose-limiting anemia and thrombocytopenia, reducing their utility for many patients (Verstovsek et al. 2012). KER-050 is an investigational product designed to alter signaling of TGF-β superfamily ligands, restoring balance to the osteohematopoietic niche and promoting differentiation of erythroid and megakaryocytic precursors. It is a modified activin receptor type IIA ligand trap that inhibits TGF-β superfamily ligands, including activins A and B and growth and differentiation factors (GDFs) 8 and 11. Preclinical studies have demonstrated that KER-050 induces erythropoiesis, increases platelet counts, improves bone mass, and prevents fibrosis. A preclinical study previously presented demonstrated that RKER-050 can reverse ruxolitinib-associated reductions in hemoglobin, hematocrit, and red blood cell count, indicating potential for KER-050 to mitigate ruxolitinib toxicity. In two clinical studies, including an ongoing study in participants with myelodysplastic syndromes (MDS), KER-050 has been well-tolerated. Accumulated clinical data demonstrate that KER-050 can stimulate erythropoiesis and thrombopoiesis in both healthy post-menopausal women and MDS participants with ineffective hematopoiesis. Here we present data from the first clinical study of KER-050 in participants with MF (NCT05037760). The objective of this study is to evaluate the potential of KER-050 as monotherapy and in combination with ruxolitinib to treat the underlying pathology of MF and mitigate JAK inhibitor-associated cytopenias. Methods: This ongoing Phase 2 open-label study is evaluating the safety, tolerability, pharmacokinetics PK), pharmacodynamics (PD), and efficacy of KER-050 administered with or without ruxolitinib in participants with primary myelofibrosis (PMF), post-essential thrombocythemia myelofibrosis (post-ET MF), and post-polycythemia vera myelofibrosis (post-PV MF) who have anemia (hemoglobin ≤ 10 g/dL or receiving transfusions). Part 1 involves parallel dose escalation arms (1A: monotherapy, 1B: combination with ruxolitinib) to evaluate the safety and tolerability of KER-050 and identify the dose(s) of KER-050 to be evaluated in Part 2 (Figure 1). Changes in hemoglobin, transfusion burden, spleen size, the Myelofibrosis Symptom Assessment Form Total Symptom Score (MF-SAF-TSS), PK and incidence of accelerated MF and acute myeloid leukemia are assessed as secondary endpoints. This study is also evaluating markers of multilineage hematopoiesis, bone remodeling, iron metabolism, inflammation, and marrow fibrosis. Frequency of ruxolitinib dose modification and quality of life will be explored. Results: Preliminary results from Part 1 will be presented. At the time of this writing, 5 patients have received KER-050 on study, 3 in Arm 1A and 2 in Arm 1B. Three participants have completed the dose-limiting toxicity (DLT) assessment period and no DLTs have been experienced. Data evaluated to date indicate a PD effect consistent with previous preclinical and clinical studies of KER-050. Summary: KER-050 is designed to alter TGF-β superfamily signaling, restoring balance to the osteohematopoietic niche and promoting multilineage hematopoietic differentiation. Here we present data from the first study of KER-050 in participants with MF evaluating the potential for KER-050 to impact key aspects of the MF disease state (anemia, spleen size, symptoms, progression), as monotherapy and in combination with ruxolitinib. The potential for KER-050 to mitigate ruxolitinib toxicity and related dose-reductions is being investigated. Exploratory analyses of the KER-050 impact on inflammation, bone remodeling, iron metabolism, and marrow fibrosis, as available, will be shared. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Abstract 11565: Improvement in Cardiopulmonary Function in a Rat Model of Pulmonary Arterial Hypertension Observed With RKER-012, a Novel Activin Receptor Type II Ligand Trap, was Associated With Reduced Markers of Inflammation and Fibrosis in the Right Ventricle

Pulmonary arterial hypertension (PAH) is characterized by elevated pulmonary vascular resistance, impaired cardiac output, and right ventricle (RV) overload. PAH is associated with imbalanced TGF-ß signaling, marked by elevated SMAD2/3 signaling, which is associated with inflammation and fibrosis contributing to heart failure. Initially, the RV can compensate for increased pressure due to its adaptive contractility, but chronic pressure overload leads to maladaptive RV remodeling through inflammatory and fibrotic pathways. Current therapies for PAH are primarily vasodilators which do not treat the underlying pathology. RKER-012 is a research form of KER-012, an investigational modified ActRIIB ligand trap designed to inhibit signaling through SMAD2/3. We investigated RKER-012’s mechanism of prevention of cardiac pathology in a Sugen/Hypoxia (SH) PAH model. Rats received a subcutaneous (SQ) dose of 25 mg/kg Sugen 5416 under hypoxic conditions. For 3 weeks, SH rats received vehicle (SH-VEH) or 10 mg/kg RKER-012 (SH-RKER-012) twice weekly SQ. Control rats received VEH twice weekly in normoxic conditions. Systolic pulmonary arterial pressure (sPAP) and Fulton index (FI) were measured. qPCR of RV tissue was used to evaluate the mechanism of RKER-012 in this model. SH-VEH rats had increased FI and sPAP (both p≤0.0001) compared to NX, consistent with development of cardiac and pulmonary impairment. Relative to SH-VEH, RKER-012 reduced increased FIand prevented increased sPAP (both p≤0.001) in SH rats so that sPAP did not differ from NX. Relative to NX, SH-VEH had increased RV expression of fibrotic markers such as Timp1, Col1a1, Col3a1, Runx2, and LOX (all p≤0.05). In contrast, SH-RKER-012 rats had reduced expression of Timp1, Col1a1, Col3a1, Runx2 and LOX (all p≤0.05) relative to SH-VEH. Inflammation markers CD68 and MCP1 (both p≤0.05) were increased in SH-VEH compared to NX. CD68 expression was reduced in SH-RKER-012 (p≤0.01) compared to NX but MCP1 expression was not affected (p=0.54). These results support that RKER-012 attenuated upregulation of key inflammation and fibrosis markers in a PAH preclinical model and suggest that KER-012 has a distinct mechanism from vasodilation and could be developed as a cardioprotective treatment for PAH.

RF28 | PSUN198 The Dramatic Fat Mass Loss Caused by Bimagrumab is Similar in Diabetic and Non-diabetic Patients.

Abstract Bimagrumab is a fully human, ligand-blocking antibody to activin receptors type IIA and type IIB that in obese diabetic patients has been shown to improve body composition dramatically. After 48 weeks of treatment (N=75, allocated 1: 1, bimagrumab intravenously (iv) 10 mg/kg/month: placebo), in the bimagrumab group total body fat mass decreased by 20.5% while at the same time, lean mass increased by 3.6%, both assessed by dual X-ray absorptiometry (DXA). At 24 weeks, fat mass loss was 16.5% in those treated with bimagrumab, while there was little effect on fat or lean mass in the placebo group at either timepoint. In a subset of patients who underwent magnetic resonance imaging, the fat loss effect was greatest for hepatic fat, with a 52% reduction from baseline to week 48, and for abdominal visceral fat, where a 34% decrease was observed over the same interval (Heymsfield SB et al. JAMA Open 2021;4: e2033457). Most drugs for obesity are less effective in diabetics, so to evaluate the efficacy of bimagrumab on fat mass in non-diabetics, we undertook a pooled analysis of patient-level data from 6 clinical studies of bimagrumab previously conducted to assess muscle anabolic activity and/or clinical safety. Criteria for study selection included at least 24 weeks of therapeutic drug exposure, DXA performed at baseline and at least 2 additional time points. Studies that recruited patients with known, underlying primary diseases of adipose tissue or muscle were excluded. Of the 568 subjects in these 6 studies, 7 were identified as diabetic and excluded, leaving 561 subjects in 4 dose groups including placebo (N=204), 70 mg iv monthly (N=43), 210 mg iv monthly (N=75), or a top dose group comprising 700 mg iv once monthly or 10 mg/kg iv once monthly or 30 mg/kg iv q8w x 2 (total N=239). A dose- and time-dependent effect of bimagrumab on fat loss was observed and at the 24 week time point, the top dose group lost 14.6% (LS mean (95% CI) 13.2% - 16.0%) of total body fat mass, while the placebo group increased their fat mass by 2.4% (0.9% - 3.8%). Approximately 41% of the subjects in this pooled analysis were overweight or obese, which is a potential limitation of this study since all subjects in the diabetes study were overweight or obese, however this likely does not affect the conclusion since no effect of baseline BMI on % initial body fat lost at week 24 was seen. Thus, unlike most other drugs for obesity, the efficacy of bimagrumab is very similar in both diabetic patients and in non-diabetic patients. Presentation: Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m., Monday, June 13, 2022 12:58 p.m. - 1:03 p.m.

Changes in Ovulation-Related Gene Expression during Induced Ovulation in the Amur Sturgeon (Acipenser schrenckii) Ovarian Follicles.

The luteinizing hormone (LH) and maturation-inducing steroids (MIS), such as 17α,20β-dihydroxy-4-pregnen-3-one, regulate the final oocyte maturation in teleosts. Oocyte maturational competence (OMC) and ovulatory competence measure the sensitivity to MIS for oocyte maturation and ovulation, respectively. However, the molecular mechanisms underlying the acquisition of ovulatory competence remain unknown. Sturgeons are an excellent research model for investigating these mechanisms. We examined the seasonal profiles of OMC and ovulatory competence in vitro and the expression of 17 ovulation-related gene candidates using quantitative PCR in Amur sturgeon ovarian follicles. The ovulatory competence was induced by the LH-releasing hormone analog (LHRHa) priming injection after acquiring the OMC, which was spontaneously induced in spring or autumn. Seven genes, including the tissue-type plasminogen activator (plat), were enhanced following the LHRHa priming injection in ovarian follicles sampled from anovulated and ovulated fish. The activin receptor type 1 (acvr1) and prostaglandin G/H synthase 2 (ptgs2) were only upregulated in ovulated fish. Our results suggest that plat/plasmin and prostaglandin (PG)/PG receptor systems are essential for sturgeon ovulation, similar to other vertebrates. Notably, successful ovulation depends on a sufficient PG synthesis, and mediators activating the PG/PG receptor system are essential for acquiring the ovulatory competence. We provide the first report of ovulation-related gene alterations in the ovarian follicles of Amur sturgeons.

Detection of activin receptor type IIA and IIB-Fc fusion proteins by automated capillary immunoassay.

Activin receptor type IIA and type IIB fusion protein have been designed to sequester circulating molecules of the transforming growth factor-β (TGF-β) superfamily and inactivate their actions. Members of this superfamily have been reported as essential regulators of erythropoiesis by triggering the formation of activated ternary complexes containing different combinations of type I and type II receptors, which can limit RBC production by accelerating erythroid differentiation and inhibiting erythroid progenitor expansion. The recent approval of Luspatercept for the treatment of anemia associated to transfusion-dependent MDS and Beta-thalassemia in afflicted patients means that it can now pose a real threat of being abused in sport for its ability to stimulate erythropoiesis. Several methods for the detection of these molecules in blood have been proposed for the purpose of sport antidoping control. Here we propose the detection of the ActRIIA-Fc and ActRIIB-Fc fusion proteins by automated capillary Western immunoassay (Simple Western). The use of these immunoassays for the detection of protein targets has become widespread in the recent years. The work presented here demonstrates that this methodology enables a versatile, rapid, and sensitive detection of activin ligand traps in blood samples: plasma, serum, or dried blood spots (DBS). Preliminary results indicate that detection in urine samples is also possible. The option to use different antibodies allows the possibility to use this method as an initial testing procedure as well as a confirmation procedure. Finally, results coming from an administration study confirm that the method is suitable for routine analysis.

Luspatercept: A New Tool for the Treatment of Anemia Related to β-Thalassemia, Myelodysplastic Syndromes and Primary Myelofibrosis.

Anemia is a common feature of both benign and malignant hematologic diseases. Beta-thalassemia (β-thalassemia) syndromes are a group of hereditary disorders characterized by ineffective erythropoiesis, due to a genetic deficiency in the synthesis of the beta chains of hemoglobin, often accompanied by severe anemia and the need for red blood cell (RBC) transfusions. Myelodysplastic syndromes (MDS) are characterized by cytopenia(s) and ineffective hematopoiesis, despite a hypercellular bone marrow. Primary myelofibrosis (PMF) is a clonal myeloproliferative neoplasm characterized by reactive fibrosis of the bone marrow, accompanied by extramedullary hematopoiesis. Luspatercept, previously known as ACE-536, is a fusion protein that combines a modified activin receptor IIB (ActRIIB), a member of the transforming growth factor-β (TGF-β) superfamily, with the Fc domain of human immunoglobulin G (IgG1). It has shown efficacy in the treatment of anemia due to beta β-thalassemia, MDS and PMF and recently gained approval by the Federal Drug Agency (FDA) and the European Medicines Agency (EMA) for transfusion-dependent (TD) patients with β-thalassemia and very low to intermediate-risk patients with MDS with ringed sideroblasts who have failed to respond to, or are ineligible for, an erythropoiesis-stimulating agent. In this review, we describe the key pathways involved in normal hematopoiesis and the possible mechanism of action of luspatercept, present its development and data from the most recent clinical trials in β-thalassemia, MDS and PMF, and discuss its potential use in the treatment of these hematological disorders.

Regulation of Nodal signaling propagation by receptor interactions and positive feedback

During vertebrate embryogenesis, the germ layers are patterned by secreted Nodal signals. In the classical model, Nodals elicit signaling by binding to a complex comprising Type I/II Activin receptors (Acvr) and the co-receptor Tdgf1. However, it is currently unclear whether receptor binding can also affect the distribution of Nodals themselves through the embryo, and it is unknown which of the putative Acvr paralogs mediate Nodal signaling in zebrafish. Here, we characterize three Type I (Acvr1) and four Type II (Acvr2) homologs and show that - except for Acvr1c - all receptor-encoding transcripts are maternally deposited and present during zebrafish embryogenesis. We generated mutants and used them together with combinatorial morpholino knockdown and CRISPR F0 knockout (KO) approaches to assess compound loss-of-function phenotypes. We discovered that the Acvr2 homologs function partly redundantly and partially independently of Nodal to pattern the early zebrafish embryo, whereas the Type I receptors Acvr1b-a and Acvr1b-b redundantly act as major mediators of Nodal signaling. By combining quantitative analyses with expression manipulations, we found that feedback-regulated Type I receptors and co-receptors can directly influence the diffusion and distribution of Nodals, providing a mechanism for the spatial restriction of Nodal signaling during germ layer patterning.

The activin receptor ligand trap ActRIIB:ALK4-Fc ameliorates cardiomyopathy induced by neuromuscular disease and diabetes.

Cardiomyopathies are ascribed to a variety of etiologies, present with diverse clinical phenotypes, and lack disease-modifying treatments. Mounting evidence implicates dysregulated activin receptor signaling in heart disease and highlights inhibition of this pathway as a potential therapeutic target. Here, we explored the effects of activin ligand inhibition using ActRIIB:ALK4-Fc, a heterodimeric receptor fusion protein, in two mechanistically distinct murine models of cardiomyopathy. Treatment with ActRIIB:ALK4-Fc significantly improved systolic or diastolic function in cardiomyopathy induced by neuromuscular disease or diabetes mellitus. Moreover, ActRIIB:ALK4-Fc corrected Ca2+ handling protein expression in diseased heart tissues, suggesting that activin signaling inhibition could alleviate cardiomyopathies in part by rebalancing aberrant intracellular Ca2+ homeostasis-a common underlying pathomechanism in diverse heart diseases.