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P-glycoprotein (Pgp; also known as MDR1, ABCB1) is the most important and best studied efflux transporter at the blood-brain barrier (BBB); however, the organization of Pgp is unknown. The aim of this study was to employ the recently developed super-resolution fluorescence microscopy method spectral precision distance microscopy/spectral position determination microscopy (SPDM) to investigate the spatial distribution of Pgp in the luminal plasma membrane of brain capillary endothelial cells. Potential disturbing effects of cell membrane curvatures on the distribution analysis are addressed with computer simulations. Immortalized human cerebral microvascular endothelial cells (hCMEC/D3) served as a model of human BBB. hCMEC/D3 cells were transduced with a Pgp-green fluorescent protein (GFP) fusion protein incorporated in a lentivirus-derived vector. The expression and localization of the Pgp-GFP fusion protein was visualized by SPDM. The limited resolution of SPDM in the z-direction leads to a projection during the imaging process affecting the appeared spatial distribution of fluorescence molecules in the super-resolution images. Therefore, simulations of molecule distributions on differently curved cell membranes were performed and their projected spatial distribution was investigated. Function of the fusion protein was confirmed by FACS analysis after incubation of cells with the fluorescent probe eFluxx-ID Gold in absence and presence of verapamil. More than 112,000 single Pgp-GFP molecules (corresponding to approximately 5,600 Pgp-GFP molecules per cell) were detected by SPDM with an averaged spatial resolution of approximately 40 nm in hCMEC/D3 cells. We found that Pgp-GFP is distributed in clustered formations in hCMEC/D3 cells while the influence of present random cell membrane curvatures can be excluded based on the simulation results. Individual formations are distributed randomly over the cell membrane.

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Genetic types of α1‐antitrypsin (protease inhibitor types, or Pi types) were determined in 108 patients with rheumatoid arthritis. These patients were selected for severely destructive disease and had classical rheumatoid arthritis according to ARA criteria, were seropositive, and had joint erosions shown by X‐ray. Heterozygotes for the deficiency Z allele (Pi types MZ, SZ, etc.) were found among 9.2 % of patients and 3.5 % of a control adult population. The increased frequency in patients was statistically significant. Heterozygotes were most frequent among female patients with an early onset of disease. Heterozygosity for α1‐antitrypsin deficiency may be a factor in familial recurrence of rheumatoid arthritis. Among 98 patients with juvenile rheumatoid arthritis not selected for severity, 4.1 % were Z heterozygotes compared with 1.3 % of control children, not a statistically significant difference. Reduced concentrations of α1‐antitrypsin in Z heterozygotes may be inadequate to inhibit the proteolytic enzymes released into the joints of adults with rheumatoid arthritis during phagocytosis of immune complexes. This may be a factor promoting severe joint destruction.

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