Metabolomic and molecular insights into sickle cell disease and innovative therapies

Abstract

Sickle cell disease (SCD) is an autosomal-recessive hemolytic disorder with high morbidity and mortality. The pathophysiology of SCD is characterized by the polymerization of deoxygenated intracellular sickle hemoglobin, which causes the sickling of erythrocytes. The recent development of metabolomics, the newest member of the "omics" family, has provided a powerful new research strategy to accurately measure functional phenotypes that are the net result of genomic, transcriptomic, and proteomic changes. Metabolomics changes respond faster to external stimuli than any other "ome" and are especially appropriate for surveilling the metabolic profile of erythrocytes. In this review, we summarize recent pioneering research that exploited cutting-edge metabolomics and state-of-the-art isotopically labeled nutrient flux analysis to monitor and trace intracellular metabolism in SCD mice and humans. Genetic, structural, biochemical, and molecular studies in mice and humans demonstrate unrecognized intracellular signaling pathways, including purinergic and sphingolipid signaling networks that promote hypoxic metabolic reprogramming by channeling glucose metabolism to glycolysis via the pentose phosphate pathway. In turn, this hypoxic metabolic reprogramming induces 2,3-bisphosphoglycerate production, deoxygenation of sickle hemoglobin, polymerization, and sickling. Additionally, we review the detrimental role of an impaired Lands' cycle, which contributes to sickling, inflammation, and disease progression. Thus, metabolomic profiling allows us to identify the pathological role of adenosine signaling and S1P-mediated erythrocyte hypoxic metabolic reprogramming and hypoxia-induced impaired Lands' cycle in SCD. These findings further reveal that the inhibition of adenosine and S1P signaling cascade and the restoration of an imbalanced Lands' cycle have potent preclinical efficacy in counteracting sickling, inflammation, and disease progression.

Graphical abstract

Figure 1.

The discovery of pathogenic nature of elevated plasma adenosine signaling via ADORA2B receptor activation in sickling reveals multiple innovative therapeutic targets for SCD. CD73-dependent elevated extracellular adenosine signaling via ADORA2B receptor in RBCs leads to activation of AMPK, subsequently increased BPG mutase activity, elevated 2,3-BPG production, and, eventually, increased deoxyHbS polymerization and sickling. Thus, CD73–ADORA2B–AMPK signaling cascades are innovative therapeutic targets to counteract sickling. ATP, adenosine triphosphate.

Figure 2.

Excess extracellular adenosine signaling via ADORA2B receptor contributes to priapism, penile fibrosis, and pain in SCD mice. (A) Excess extracellular adenosine activates ADORA2B receptor in corpus cavernosum and smooth muscle relaxation, contributing to penile fibrosis and priapism, respectively, in SCD. (B) Excess extracellular adenosine activates ADORA2B receptor to induce secretion of IL-6 and sIL-6R from myeloid cells. IL-6 and sIL-6R form a complex that can transactivate gp130 in DRG neurons. gp130 activation leads to activation of phosphorylated STAT3 (pSTAT3), which, in turn, increases TRPV1 gene expression in DRG neurons and overall nociception in SCD mice. ADORA2B–IL-6–sIL-6–gp130 signaling networks are innovative therapeutic targets for priapism and chronic pain. cGMP, guanosine 3′,5′-cyclic monophosphate; PDE, phosphodiesterase.

Figure 3.

The discovery of an elevation of S1P and an impaired Lands' cycle in erythrocytes promoting sickling reveals multiple potential therapeutics to treat SCD. (A) Elevated plasma adenosine signaling via ADORA2B receptor underlies activation of SphK1 and increased S1P production in sickle RBCs in a PKA- and extracellular signal-regulated kinase (ERK)–dependent manner. Elevated intracellular S1P directly binds to deoxyHbS, which promotes deoxyHbS anchoring to membrane-bound band 3. This enhances the release of membrane-bound glycolytic enzymes to the cytosol, inducing metabolic reprogramming by accelerating glucose metabolism toward glycolysis instead of PPP, which induces 2,3-BPG production and nicotinamide adenine dinucleotide phosphate–mediated glutathione synthesis reduction. As such, elevated SphK1 mediates intracellular S1P production in sickle cells, which results in an increase of 2,3-BPG production and reactive oxygen species (ROS) generation. (B) Elevated erythrocyte membrane lysophosphatidylcholine (LysoPC) content and circulating erythrocyte arachidonic acid in sickle RBCs is the result of an impaired Lands’ cycle. Correcting an imbalanced Lands’ cycle by interfering with the activity of cytosolic PLA2 or inducing activation of lysophosphatidycholine acyltransferase 1 (LPCAT1), 2 key enzymes of the Lands’ cycle, led to a reduction of elevated RBC membrane LysoPC content and circulating arachidonic acid levels, which attenuated sickling. (C) Elevated plasma S1P signaling via S1PR1 contributes to a sustained inflammatory response and disease progression in SCD by reciprocal upregulation of IL-6 and S1PR1 expression in macrophages.