RBC eNOS Transduces Regional O2 Gradients to Orchestrate RBC O2 Delivery Phenotype during Circulatory Transit

BACKGROUND: The role of endogenous endothelial NO synthase (eNOS) in mature RBCs is poorly understood. We have previously demonstrated functionally O2-dependent eNOS migration in RBCs, mediating (via s-nitrosylation) glycolytic metabolon assembly/disassembly during circulation in support of antioxidant systems during stress. We have further explored eNOS migration and now propose the following mechanism along with its functional significance. We (and others) suggest that hemoglobin (Hb) conformational change resulting from deoxygenation leads to its docking at the cytoplasmic domain of Band 3 (cdB3). We propose that (1) cdB3 consequently undergoes a conformational change that is transduced by Piezo1 (a mechanosensitive ion channel located in the RBC membrane); (2) Piezo1 activation results in calcium entry into the RBC, which (3) aids in the disassembly of the RBC cytoskeleton, (4) facilitates eNOS migration to the membrane (where calmodulin is mainly localized - thereby bringing these two important proteins together), and (5) binds to calmodulin, activating eNOS to generate nitric oxide (NO) - which then influences RBC phenotype that facilitates O ₂ delivery, specifically related to RBC deformability and regulation of Hb~O ₂ release, via enhanced Bohr effect.

METHODS: Heparinized human, eNOS ^(-/-), or wt murine (C57BL/6J) red blood cells (RBCs) were imaged (Nikon) for the analysis of cytoskeletal protein arrangement and eNOS migration. RBCs were subjected to oxygenation/deoxygenation, with/without pre-treatment with the Piezo1 agonist Yoda1, or a membrane tension buffer grammostola mechanotoxin #4 (GSMTx4; decoupling Piezo1), or carbon monoxide (CO) to maintain R-state Hb conformation upon RBC deoxygenation. RBCs were fixed (paraformaldehyde 4%/glutaraldehyde 0.08%), permeabilized (Triton X100 amount 0.1%) and incubated with primary antibodies (spectrin, actin, protein 4.1, or eNOS), then with the secondary antibody prior to visualization. Interaction between eNOS and cytoskeleton proteins, as well as calmodulin, was determined using a proximity ligation assay (Duolink). Intracellular RBC calcium levels were assessed in oxygenated/deoxygenated RBCs utilizing an environmentally controlled fluorescent plate reader, and the calcium probe Fluo 3. NO production in RBCs was assessed using DAF-FM. To assess the functional significance of this mechanism, Hb-O ₂ affinity was determined from murine blood (eNOS ^(-/-) or wt). Oxygen dissociation curves (ODCs), were constructed at 2 fixed CO ₂ levels (thin film rotating tonometer; in conjunction with a gas blender; with samples measured by arterial blood gas machine).Bohr effect was assessed by determining the area between the ODCs (ABC) at the 2 fixed CO2 levels (~40mmHg and ~70mmHg).

RESULTS: We observed several significant findings related to red RBC deoxygenation. Upon RBC deoxygenation, there is an increase in calcium levels indicated by the Fluo3 signal, which is Piezo1 dependent. Cytoskeleton proteins, such as Actin, Spectrin, and protein 4.1, undergo disassembly upon RBC deoxygenation. This disassembly process is contingent on both Piezo1-mediated calcium flux and the conformational changes in hemoglobin (Hb) (and not pO ₂). We found that eNOS is associated with 4.1 throughout the oxy/deoxy cycle and that eNOS exhibits the same migratory pattern as cytoskeleton proteins to the RBC membrane, where calmodulin is located, irrespective of the RBC's oxygenation state. Additionally, when eNOS localizes in the membrane, it exhibits interaction with calmodulin. The generation of nitric oxide (NO) by eNOS, as indicated by the DAF-FM signal, increases in a manner dependent on both calcium, Piezo1 and Hb conformational transition. The functional consequence of this mechanism appears to be related to O ₂ delivery phenotype during circulatory transit, with cell deformability being affected in addition to an inhibition of the Bohr effect in eNOS ^(-/-) compared to wt mice (2006 ± 61 vs 1892 ± 69 ABC in WT (n = 3) vs eNOS ^(-/-) (n = 4), respectively; p = 0.0747).

SUMMARY: Upon circulatory transit, RBC deoxygenation triggers the activation of eNOS. This activated eNOS is associated with Piezo1-mediated calcium flux, which is facilitated by calmodulin. These events are coordinated with the migration of eNOS within the red blood cell between the cytosol and the cell membrane. The migration of eNOS is influenced by cytoskeleton proteins.