A moonlighting job for α-globin in blood vessels

Abstract

Red blood cells express high levels of hemoglobin A tetramer (α2β2) to facilitate oxygen transport. Hemoglobin subunits and related proteins are also expressed at lower levels in other tissues across the animal kingdom. Physiological functions for most nonerythroid globins likely derive from their ability to catalyze reduction-oxidation (redox) reactions via electron transfer through heme-associated iron. An interesting example is illustrated by the recent discovery that α-globin without β-globin is expressed in some arteriolar endothelial cells (ECs). α-globin binds EC nitric oxide (NO) synthase (eNOS) and degrades its enzymatic product NO, a potent vasodilator. Thus, depletion of α-globin in ECs or inhibition of its association with eNOS causes arteriolar relaxation and lowering of blood pressure in mice. Some of these findings have been replicated in isolated human blood vessels, and genetic studies are tractable in populations in which α-thalassemia alleles are prevalent. Two small studies identified associations between loss of α-globin genes in humans and NO-regulated vascular responses elicited by local hypoxia-induced blood flow or thermal stimulation. In a few larger population-based studies, no associations were detected between loss of α-globin genes and blood pressure, ischemic stroke, or pulmonary hypertension. In contrast, a significant positive association between α-globin gene copy number and kidney disease was detected in an African American cohort. Further studies are required to define comprehensively the expression of α-globin in different vascular beds and ascertain their overall impact on normal and pathological vascular physiology.

Graphical abstract

Figure 1.

The MEJ facilitates communication between vascular ECs and smooth muscle cells to regulate vascular tone. Upon adrenergic stimulation of vascular smooth muscle, inositol triphosphate (IP3) generated by phospholipase C diffuses into ECs via gap junctions and triggers release of Ca²⁺ from the endoplasmic reticulum. Cytosolic Ca²⁺-mediated signaling leads to phosphorylation and activation of eNOS, which catalyzes the production of NO from L-arginine. NO diffuses through the MEJ into smooth muscle cells and activates guanylate cyclase, leading to an increase in cyclic guanosine monophosphate (cGMP), causing smooth muscle relaxation and vasodilation. EC α-globin can bind and degrade NO to limit its bioavailability. The figure was created using BioRender.

Figure 2.

Molecular model for α-globin function in the MEJ. In the presence of O₂, Fe²⁺ α-globin bound to eNOS degrades NO by dioxygenation, thereby enhancing vascular tone. eNOS and/or CYB5R3/CYB5A can reduce Fe³⁺ α-globin to the Fe²⁺ state to support another round of dioxygenation. Alternatively, under oxidizing conditions, Fe³⁺ α-globin may be released from eNOS and bind α-hemoglobin stabilizing protein (AHSP) to form a stable complex that cannot catalyze dioxygenation. When reducing conditions are restored, CYB5R3/CYB5A or eNOS can convert AHSP-bound Fe³⁺ α-globin to the Fe²⁺ form, which favors its transfer to eNOS for continued NO scavenging. During hypoxia, deoxy-Fe²⁺ α-globin can reduce NO₂⁻ to generate NO, thereby enhancing vasodilation. The figure was created using BioRender.

Figure 3.

The human and mouse extended α-globin loci. (A) Human and (B) mouse α-globin gene clusters are diagrammed. The major adult-expressed genes are indicated with blue font. Corresponding transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) peaks indicate open chromatin in adult RBC precursors. Enhancer modules associated with open chromatin (ATAC-seq signal) are indicated as multispecies conserved sequence (MCS) R1-R4 (human) or R1, R2, R3, Rm, and R4 (mouse). Human HBA2 is nearest to the enhancer and is expressed at twofold- to fivefold-higher levels than HBA1 in adult RBCs. Similarly, mouse Hba-a1 is nearest to the enhancer and is expressed at twofold- to threefold-higher levels than Hba-a2 in adult RBCs. The common α-thalassemia deletion, −α3.7 shown at the bottom of panel A generates a single α-globin fusion gene.