Hypoxic vasodilatory defect and pulmonary hypertension in mice lacking hemoglobin β-cysteine93 S-nitrosylation

Abstract

Systemic hypoxia is characterized by peripheral vasodilation and pulmonary vasoconstriction. However, the system-wide mechanism for signaling hypoxia remains unknown. Accumulating evidence suggests that hemoglobin (Hb) in RBCs may serve as an O 2 sensor and O 2 -responsive NO signal transducer to regulate systemic and pulmonary vascular tone, but this remains unexamined at the integrated system level. One residue invariant in mammalian Hbs, β-globin cysteine93 (βCys93), carries NO as vasorelaxant S -nitrosothiol (SNO) to autoregulate blood flow during O 2 delivery. βCys93Ala mutant mice thus exhibit systemic hypoxia despite transporting O 2 normally. Here, we show that βCys93Ala mutant mice had reduced S -nitrosohemoglobin (SNO-Hb) at baseline and upon targeted SNO repletion and that hypoxic vasodilation by RBCs was impaired in vitro and in vivo, recapitulating hypoxic pathophysiology. Notably, βCys93Ala mutant mice showed marked impairment of hypoxic peripheral vasodilation and developed signs of pulmonary hypertension with age. Mutant mice also died prematurely with cor pulmonale (pulmonary hypertension with right ventricular dysfunction) when living under low O 2 . Altogether, we identify a major role for RBC SNO in clinically relevant vasodilatory responses attributed previously to endothelial NO. We conclude that SNO-Hb transduces the integrated, system-wide response to hypoxia in the mammalian respiratory cycle, expanding a core physiological principle.

Nitric oxide signaling in health and disease

“…Multiple different experiments both in vivo and in vitro now show that NO bioactivity export from RBCs does not require the presence of a b93 cysteine in Hb, leaving the exact mechanism of the RBCderived vasodilatation and tissue protection unresolved (Isbell et al, 2008;Sun et al, 2019). Although these mice clearly have a phenotype with pulmonary hypertension and premature death when kept in a low oxygen environment (Zhang et al, 2022), there is no evidence that this is selectively caused by abrogated SNO-Hb signaling. Notably, the Hb b93 cysteine might in fact have an alternative role for example in protecting the heme iron against oxidation as suggested in early studies (Winterbourn and Carrell, 1977).…”

S‐Nitroso‐l‐cysteine and ventilatory drive: A pediatric perspective

“…Of these, GSNOR appears to be the most active in respiratory control, and mice missing this enzyme have prolonged neuronal exposure to GSNO (and therefore to downstream l ‐CSNO) and have sustained hyperventilation during and after hypoxia (Figure 2). 24,47 Note in this regard that the Hb beta93 −/− the mouse has dramatically impaired posthypoxic facilitation (Figure 2) and, in general, an imparied response to hypoxia 58 . Additionally, the nNOS‐deficient mouse also has an impaired ventilatory response to hypoxia 57 .…”

“…Moreover, cysteine esters, bypassing the LAT transporter for access to intracellular targets, are being developed as respiratory stimulants that are safe and effective in animals. 58,70 D-isomers with access to the intracellular space are effective and do not appear to have the side effects of the L-isomer-based esters. 71 These compounds could be among the first ever effective respiratory stimulants for a variety of causes of respiratory depression in children, including respiratory depressant narcotic and benzodiazepine use in the pediatric critical care setting.…”

“…It has not previously been considered that acute use of NAC could be studied as a clinical substitute for caffeine in apnea of prematurity. Moreover, cysteine esters, bypassing the LAT transporter for access to intracellular targets, are being developed as respiratory stimulants that are safe and effective in animals 58,70 . D‐isomers with access to the intracellular space are effective and do not appear to have the side effects of the l ‐isomer‐based esters 71 .…”

“…24,47 Note in this regard that the Hb beta93 −/− the mouse has dramatically impaired posthypoxic facilitation (Figure 2) and, in general, an imparied response to hypoxia. 58 Additionally, the nNOSdeficient mouse also has an impaired ventilatory response to hypoxia. 57 Further, mice lacking GGT have a profoundly impairedindeed, paradoxicalresponse to hypoxia, likely because they cannot bioactivate GSNO by forming L-CSNO (Figures 1 and 2).…”