Protein quality control during erythropoiesis and hemoglobin synthesis

Abstract

Erythrocytes must regulate hemoglobin synthesis to limit the toxicities of unstable free globin chain subunits. This regulation is particularly relevant in β-thalassemia, in which β-globin deficiency causes accumulation of free α-globin, which forms intracellular precipitates that destroy erythroid precursors. Experimental evidence accumulated over more than 40 years indicates that erythroid cells can neutralize moderate amounts of free α-globin through generalized protein quality control mechanisms, including molecular chaperones, the ubiquitin-proteasome system, and autophagy. In many ways, β-thalassemia resembles protein aggregation disorders of the nervous system, liver, and other tissues, which occur when levels of unstable proteins overwhelm cellular compensatory mechanisms. Information gained from studies of nonerythroid protein aggregation disorders may be exploited to further understand and perhaps treat β-thalassemia.

Figure 1. Electron microscopy reveals aggresome-like α globin inclusions in β-thalassemia

(A) Several late polychromatic erythroblasts, labeled A, B, and C, with intracytoplasmic perinuclear α-chain precipitates (adapted from Wickramasinghe SN, Bush V. Observations on the ultrastructure of erythropoietic cells and reticulum cells in the bone marrow of patients with homozygous beta-thalassaemia. Br J Haematol 1975;30(4):395-399, with permission); (B) precipitation of α-chains around the centriole of an erythroblast. The arrow indicates microtubule triplets in centriole cross-section (adapted from Wickramasinghe SN, Hughes M. Precipitation of alpha-chains on the centrioles of erythroblasts in beta-thalassaemia. Br J Haematol 1982;52(4):681-682, with permission); (C) Top: Autophagosome membrane formation partially enclosing precipitated α-chains; Bottom: Two autophagic vacuoles containing electrodense material, probably precipitated α chains (adapted from Wickramasinghe SN, Hughes M. Ultrastructural studies of erythropoiesis in beta-thalassaemia trait. Br J Haematol 1980;46(3):401-407, with permission); (D) Immunogold staining for ubiquitin within α globin precipitates of a β-thalassemic erythroblast (adapted from Wickramasinghe SN, Lee MJ. Evidence that the ubiquitin proteolytic pathway is involved in the degradation of precipitated globin chains in thalassaemia. Br J Haematol 1998;101(2):245-250, with permission).

Figure 2. Model for repair or elimination of free α globin by cellular protein quality control systems

Misfolded free α globin may be stabilized by interactions with molecular chaperones, perhaps AHSP and/or generalized public chaperones. Irreversibly damaged protein may be removed either by chaperone-mediated autophagy or ubiquitin-proteasome (UPS) pathways. E1 ubiquitin-activating enzymes, E2 ubiquitin-conjugating enzymes, and E3 ubiquitin ligases mediate conjugation of polyubiquitin (Ub) chains to α globin, targeting it for proteasomal degradation via the UPS. In addition, insoluble α globin (+/- Ub) can be shuttled to aggresomes, which are subsequently cleared by macroautophagy.

Figure 3. Overloading of degradation systems leads to accumulation of α globin in β-thalassemia

During normal erythropoiesis, α and β globin subunits are synthesized close to equally and join to form HbA (α₂β₂) tetramers. In β-thalassemia trait, excess α chains are synthesized, but removed by protein quality control systems and there is minimal pathology. In β-thalassemia, free α globin is degraded in early erythroid precursors in attempts to balance globin synthesis. As cellular maturation proceeds, free α chain levels overwhelm compensatory mechanisms, resulting in accumulation of toxic precipitates. Precipitated α globin can damage the cell either directly, or indirectly by interfering with the functions of protein quality control systems.