Hyperactive deoxy-PIEZO1 shapes the circulatory life cycle of irreversibly sickled cells

Sickle cell disease (SCD), affecting millions worldwide, is caused by the homozygous inheritance of the abnormal hemoglobin HbS. Deoxygenation of HbS in the venous circulation permeabilizes sickle cells to calcium via PIEZO1 channels, triggering a dehydration cascade driven by the outward electrochemical potassium gradient. This mechanism operates with particular intensity in a subpopulation of sickle red blood cells (RBCs), the irreversibly sickled cells (ISCs). The lifespan of ISCs is extremely short, about 4-7 days. Most of this time is spent in a profoundly dehydrated condition, the irreversibly sickled state, eliciting vaso-occlusion, which is considered the root cause of organ failure and pain crisis in SCD. There is a large experimental and clinical database on sickle cells and ISCs, but how ISCs form and evolve in the circulation remains a mystery. The present study is the first attempt to unravel the experimentally inaccessible life cycle of ISCs in vivo by applying a well-accredited model of RBC homeostasis and circulatory dynamics, using a vast array of validated experimental observations to tightly constrain the model parameters. The results showed that abnormally strong deoxy-PIEZO1 responses were needed for calcium to elicit a violent hyperdense collapse in ISC-destined stress reticulocytes within about a day in the circulation. The potassium-depleted ISCs remain in this maximally dehydrated but volume-stable condition, the pathogenic state, sustained by vigorous pump-leak balanced sodium fluxes. Eventually, sodium pump decay initiates rapid terminal rehydration by the unbalanced net gain of NaCl and water. Analysis of the mechanisms behind this three-stage circulatory life cycle of ISCs exposed a complex web of interactions among many components of the homeostatic fabric of RBCs. These findings point to the abnormally intense PIEZO1 response to deoxygenation in ISC-destined stress reticulocytes as a prime cause of ISC formation in vivo, a central target for future research.