Unc 51–like autophagy-activating kinase (ULK1) mediates clearance of free α-globin in β-thalassemia

Abstract

Erythroid maturation is coordinated to maximize the production of hemoglobin A heterotetramers (α2β2) and minimize the accumulation of potentially toxic free α-or βglobin subunits. In β-thalassemia, mutations in the β-globin gene cause a build-up of free α-globin, which forms intracellular precipitates that impair erythroid cell maturation and 5 viability. Protein quality-control systems mitigate β-thalassemia pathophysiology by degrading toxic free α-globin. We show that loss of the Unc 51-like autophagy-activating kinase gene Ulk1 in β-thalassemic mice reduces autophagic clearance of α-globin in red cell precursors and exacerbates disease phenotypes, whereas inactivation of the canonical autophagy gene Atg5 has minimal effects. Systemic treatment with rapamycin 10 to inhibit the ULK1 inhibitor mTORC1 reduces α-globin precipitates and lessens pathologies in β-thalassemic mice, but not in those lacking Ulk1. Similarly, rapamycin reduces free α-globin accumulation in erythroblasts derived from β-thalassemic patient CD34 + hematopoietic progenitors. Our findings identify a new, drug-regulatable pathway for ameliorating β-thalassemia. 15 One Sentence Summary:Rapamycin alleviates -thalassemia by stimulating ULK1-dependent autophagy of toxic free -globin. 20

Interplay between α‐thalassemia and β‐hemoglobinopathies: Translating genotype–phenotype relationships into therapies

“…For instance, pharmacological activation of UPS and autophagy pathways have further exemplified the role of clearing toxic α-globin protein aggregates to alleviate the manifestation of disease. 110 Gene therapy/ RNAi/gene editing strategies have been used to alter the expression of α-globin while increasing β-globin expression and reversing the α/β-globin chain imbalance in β-thalassemia. 146,147,151 Pharmacological silencing of α-globin and induction of γ-globin gene expression has demonstrated synergistic activity without affecting erythroid cell viability and differentiation in cord blood-derived HSCs.…”

“…[107][108][109] (D) Reduced autophagic clearance of aggregated α-globin in erythroblasts induces ineffective erythropoiesis and apoptosis, whereas the induction of ULK1-mediated autophagy by rapamycin is associated with reduced hemolysis and enhanced cell survival. 110 and oxidative stress (Figure 3). [116][117][118][119] Notably, the tightly regulated UPS is also a target of oxidative stress.…”

“…124 The identification of ULK1 as a central regulator of excess α-globin has drawn attention to the mTORC1 signaling pathway and potential clinical application of mTORC1 inhibitors to enhance autophagy and decrease ineffective erythropoiesis in β-thalassemia patients with substantially residual βor γ-globin chain synthesis. 110 Additional studies have tested rapamycin in combination with RAP-536 in β-thalassaemic mice. 125 RAP-536, the murine analog of luspatercept, selectively binds transforming growth factor-β (TGF-β) superfamily ligands to reduce SMAD2/3 signaling and promote erythroid maturation.…”

“…The molecular chaperone, AHSP, binds to a basic and hydrophobic (BH) domain of free α-globin and prevents misfolding and protease digestion prior to HbA assembly, whereas unpaired or excess α-globin is selectively eliminated by the UPS (Figure 3). [110][111][112] The UPS is central to the regulation of all cellular processes, such as transcriptional regulation, signal transduction, and stress response, and defects in this system can result in several human disorders. 113 During normal erythroid differentiation, several quality control factors are upregulated, required for the degradation and elimination of various intracellular organelles.…”

“…By stimulating this pathway, rapamycin improved the terminal maturation of erythroblasts and anemia in a mouse model of β-thalassemia. 110 However, the loss of the Ulk1 gene in β-thalassaemic mice reduced autophagic clearance of excess α-globin in erythroblasts and exacerbated disease phenotypes. 110 Recent studies have shown that the bicistronic microRNA miR-144/45, which is the highest expressed miR locus in terminal erythroid cells, is a genetic modifier of β-thalassemia.…”