External PS in Sickle RBC: Relationships among Aminophospholipid Translocase (APLT), Phospholipid Scramblase (PLSCR), Cell Maturity, and Cell Density.

External phosphatidylserine (PS) is present on some sickle RBC and may contribute to thrombogenesis, endothelial adhesion, and shortened RBC lifespan. Phospholipid scramblase (PLSCR) disrupts phospholipid (PL) asymmetry by causing nonspecific PL equilibration across the membrane. Aminophospholipid translocase (APLT) maintains PL asymmetry by returning externalized PS to the inner membrane leaflet. It has been proposed that both APLT inhibition and PLSCR activation are required for PS externalization. Sickle RBC with low level external PS (Type I PS+) are present in cells of all densities and include some reticulocytes. Sickle RBC with high external PS (Type II PS+) are primarily found in the dense fraction. Type II cells are thought to be more important because:

1. the high level of external PS should have greater consequence;

2. high level external PS occurs primarily in pathologically dehydrated sickle RBC; and

3. low level external PS appears to be physiological in immature RBC.

We have previously shown that dense, dehydrated sickle RBC, including the small number of dense transferrin receptor positive (TfR+) reticulocytes, have markedly inhibited APLT. In the current studies, we examined the relationships among external PS, APLT, PLSCR, and density in mature RBC and TfR+ reticulocytes using 3-color flow cytometry. APLT and PLSCR activities were assayed using fluorescent PL analogues (NBD-PS and NBD-PC, respectively), and expressed as the fraction of probe internalized. External PS was measured with Annexin V-PE and TfR+ reticulocytes were identified with anti-TfR-PE/Cy5. PS+ cells had lower APLT activity compared to PS- cells that did not reach significance for n=3 (NBD-PS internalization fraction for PS-: 0.586±0.053; Type I PS+: 0.517±0.158, Type II PS+: 0.523±0.033). PS- sickle RBC had a uniformly low PLSCR activity similar to normal RBC (NBD-PC internalization fractions ∼ 0.1). In mature sickle RBC, PLSCR was more active in PS+ cells (PS-: 0.097±0.096; Type I PS+: 0.163±0.070, Type II PS+: 0.248±0.043; n=3; PS- vs Type I PS+: p=0.06; PS- vs Type II PS+: p=0.04; Type I versus Type II: p=0.03). TfR+ reticulocytes had increased APLT and PLSCR activity compared to mature sickle RBC, but there was no apparent relationship between PLSCR and external PS. Since dense sickle RBC had markedly inhibited APLT, we evaluated the relationship between dehydration and APLT activity. Dehydration of AA RBC from an MCHC of 35.6±2.2 to 49.2±2.0 g/dL inhibited APLT (from 0.484±0.068 to 0.301±0.076; n=7, p= 0.01). Dehydration of SS RBC from an MCHC of 34.8±3.5 to 50.1±3.9 g/dL also inhibited APLT (from 0.460±0.060 to 0.361±0.047; n=3, p=0.006), but not as low as in SS RBC dehydrated in vivo (0.222±0.036 at 44.7±5.6 g/dL; n=4, p=0.007 vs. SS RBC dehydrated in vitro). Rehydration of AA and SS RBC that had been dehydrated in vitro reversed APLT inhibition. However, APLT activity was not reversed upon rehydration of sickle RBC dehydrated in vivo. In summary, our data show that:

1. many dense sickle RBC with significantly inhibited APLT are PS-, indicating that APLT inhibition alone does not result in PS externalization;

2. dehydration contributes to, but is not entirely responsible for, the APLT inhibition seen in dense sickle RBC; and

3. PS+ sickle RBC have increased PLSCR activity.