A surface replica method: a useful tool for studies of the cytoskeletal network in red cell membranes of normal subjects and patients with a beta-spectrin mutant (spectrin Le Puy: beta 220/214)

Human erythrocytes possess a cytoplasmic endoskeleton containing .BETA.-actin and neurofilament protein

The biconcave disc shape of mammalian erythrocytes has been considered to be maintained only with a membrane underlain by a membranous cytoskeleton. Our improved ion-etching/scanning electron microscopy and saponin-ethanol treatment combined with immunocytochemistry in the human red blood cell revealed the three-dimensional structure of this cytoplasmic endoskeleton apart from the classical membranous cytoskeleton. The endoskeletal meshwork images obtained by the saponin-ethanol treatment corresponded to those by the repeated ion-etching method. The actin-rich endoskeleton was divided into two layers, one superficial and the other deep. The superficial filaments were perpendicularly connected to the membranous cytoskeleton, while the deep filaments formed an irregularly directed complicated meshwork. In the transitional hillside region between the convex periphery and concave center, the endoskeletal filaments containing a neurofilament protein ran parallel to the hillside slope toward the concave center. The endoskeleton of the erythrocyte associating with the membranous cytoskeleton may serve to keep its unique biconcave disc shape deformable, pliable, and restorable against external circumstances.

Late expression of red cell membrane protein 4.2 in normal human erythroid maturation with seven isoforms of the protein 4.2 gene

The expression of protein 4.2 in normal human erythroid cells was studied utilizing erythroblasts from bone marrow and erythroid cells cultured by the two-phase liquid culture method from burst-forming unit erythroid (BFU-E) in peripheral blood. As opposed to spectrin, which was expressed in erythroid progenitors or very early erythroblasts, protein 4.2 was first detected in late erythroblasts with a morphology nearly identical to orthochromatic erythroblasts. Among the various major membrane proteins, the expression of protein 4.2 was the latest. At the gene level, protein 4.2 gene mRNA was expressed in early erythroblasts. During normal erythroid maturation, the expression of seven different protein 4.2 gene products was observed by Southern blot analysis. These seven gene products appeared to be derived from protein 4.2 gene in the presence or absence of skipping of the 90 bp in exon 1, exon 3, and/or exon 5, as judged by deduction from the protein 4.2 sequence. Therefore, it can be speculated that protein 4.2 is expressed after the cytoskeletal network has been constructed and assembled with integral proteins in the membrane lipid bilayer.

Homozygous missense mutation (band 3 Fukuoka: G130R): a mild form of hereditary spherocytosis with near-normal band 3 content and minimal changes of membrane ultrastructure despite moderate protein 4.2 deficiency

The characteristics of phenotypic expression were studied in a Japanese family with hereditary spherocytosis and an extremely rare homozygous missense mutation of the band 3 gene (band 3 Fukuoka: G130R). The homozygous unsplenectomized proband was a 29-year-old male with compensated haemolytic anaemia (red cell count 4.21 x 10(12)/l, reticulocytes 278 x 10(9)/l, and indirect bilirubin 44 micromol/l). His red cell band 3 (B3) protein demonstrated a 9.3% reduction and his protein 4.2 (P4.2) level was substantially reduced (45.0%), compared to normal subjects. P4.2 protein was composed mostly of a wild type (72 kD) with a trace of 68 kD peptide. The binding properties of the mutated B3 to normal P4.2 were significantly impaired, which probably resulted in the substantial reduction of P4.2 in this proband, since no abnormalities were detected on the P4.2 gene. Electron microscopy (EM) using the freeze-fracture method demonstrated a mild decrease in intramembrane particles (IMPs) of near-normal size (8 nm in diameter) with no substantial increases in their oligomerization. Their distribution on the membrane P face was almost normal, although most of the IMPs could represent the homozygously mutated B3 protein. EM (quick-freeze deep-etching method) disclosed a skeletal network of near-normal size and size distribution of the skeletal units, suggesting that the mutated B3 protein itself did not have much effect on the skeletal network in situ. Therefore the reduced P4.2 content (45% of that of normal subjects), which remained on the red cell membrane of this proband, appeared to be nearly sufficient for maintaining the normal structure of the skeletal network and IMPs in situ, contrary to the marked abnormalities in both IMPs and the skeletal network in complete P4.2 deficiencies.

A Markedly Disrupted Skeletal Network With Abnormally Distributed Intramembrane Particles in Complete Protein 4.1-Deficient Red Blood Cells (Allele 4.1 Madrid): Implications Regarding a Critical Role of Protein 4.1 in Maintenance of the Integrity of the Red Blood Cell Membrane

Electron microscopic (EM) studies were performed to clarify the interactions of membrane proteins in the red blood cell membrane structure in situ of a homozygous patient with total deficiency of protein 4.1 who carried a point mutation of the downstream translation initiation codon (AUG → AGG) of the protein 4.1 gene [the 4.1 (−) Madrid; Dalla Venezia et al, J Clin Invest 90:1713, 1992]. Immunologically, as expected, protein 4.1 was completely missing in the red blood cell membrane structure in situ. A markedly disrupted skeletal network was observed by EM using the quick-freeze deep-etching method and the surface replica method, although the number of spectrin molecules was only minimally reduced (395 ± 63/μm2; normal, 504 ± 36/μm2). The number of basic units in the skeletal network was strikingly reduced (131 ± 21/μm2; normal, 548 ± 39/μm2), with decreased small-sized units (17 ± 4/μm2; normal, 384 ± 52/μm2) and increased large-sized units (64% ± 14%; normal, 5% ± 1%). Concomitantly, immuno-EM disclosed striking clustering of spectrin molecules with aggregated ankyrin molecules in the red blood cell membrane structure in situ. Although no quantitative abnormalities in the number and size distribution of the intramembrane particles were observed, there was a disappearance of regular distribution, with many clusters of various sizes, probably reflecting the distorted skeletal network. Therefore, protein 4.1 suggests by EM to play a crucial role in maintenance of the normal integrity of the membrane structure in situ not only of the skeletal network but also of the integral proteins.

A Markedly Disrupted Skeletal Network With Abnormally Distributed Intramembrane Particles in Complete Protein 4.1-Deficient Red Blood Cells (Allele 4.1 Madrid): Implications Regarding a Critical Role of Protein 4.1 in Maintenance of the Integrity of the Red Blood Cell Membrane

Electron microscopic (EM) studies were performed to clarify the interactions of membrane proteins in the red blood cell membrane structure in situ of a homozygous patient with total deficiency of protein 4.1 who carried a point mutation of the downstream translation initiation codon (AUG → AGG) of the protein 4.1 gene [the 4.1 (−) Madrid; Dalla Venezia et al, J Clin Invest 90:1713, 1992]. Immunologically, as expected, protein 4.1 was completely missing in the red blood cell membrane structure in situ. A markedly disrupted skeletal network was observed by EM using the quick-freeze deep-etching method and the surface replica method, although the number of spectrin molecules was only minimally reduced (395 ± 63/μm2; normal, 504 ± 36/μm2). The number of basic units in the skeletal network was strikingly reduced (131 ± 21/μm2; normal, 548 ± 39/μm2), with decreased small-sized units (17 ± 4/μm2; normal, 384 ± 52/μm2) and increased large-sized units (64% ± 14%; normal, 5% ± 1%). Concomitantly, immuno-EM disclosed striking clustering of spectrin molecules with aggregated ankyrin molecules in the red blood cell membrane structure in situ. Although no quantitative abnormalities in the number and size distribution of the intramembrane particles were observed, there was a disappearance of regular distribution, with many clusters of various sizes, probably reflecting the distorted skeletal network. Therefore, protein 4.1 suggests by EM to play a crucial role in maintenance of the normal integrity of the membrane structure in situ not only of the skeletal network but also of the integral proteins.

Defective anion transport and marked spherocytosis with membrane instability caused by hereditary total deficiency of red cell band 3 in cattle due to a nonsense mutation

We studied bovine subjects that exhibited a moderate uncompensated anemia with hereditary spherocytosis inherited in an autosomal incompletely dominant mode and retarded growth. Based on the results of SDS-PAGE, immunoblotting, and electron microscopic analysis by the freeze fracture method, we show here that the proband red cells lacked the band 3 protein completely. Sequence analysis of the proband band 3 cDNA and genomic DNA showed a C --> T substitution resulting in a nonsense mutation (CGA --> TGA; Arg --> Stop) at the position corresponding to codon 646 in human red cell band 3 cDNA. The proband red cells were deficient in spectrin, ankyrin, actin, and protein 4.2, resulting in a distorted and disrupted membrane skeletal network with decreased density. Therefore, the proband red cell membranes were extremely unstable and showed the loss of surface area in several distinct ways such as invagination, vesiculation, and extrusion of microvesicles, leading to the formation of spherocytes. Total deficiency of band 3 also resulted in defective Cl-/HCO3- exchange, causing mild acidosis with decreases in the HCO3- concentration and total CO2 in the proband blood. Our results demonstrate that band 3 indeed contributes to red cell membrane stability, CO2 transport, and acid-base homeostasis, but is not always essential to the survival of this mammal.

Electron microscopic evidence of impaired intramembrane particles and instability of the cytoskeletal network in band 4.2 deficiency in human red cells

To obtain direct evidence of impaired intramembrane particles (IMPs) and a deranged cytoskeletal network in situ in human red cells of band 4.2 deficiency, electron microscopic studies were performed utilizing the freeze fracture method for IMPs and the quick-freeze deep-etching method for the cytoskeletal network. Three patients with three different previously identified mutations of the band 4.2 gene, i.e., band 4.2 Komatsu (homozygous; codon 175 GAT --> TAT), band 4.2 Nippon (homozygous; codon 142 GCT --> ACT), and band 4.2 Shiga (compound heterozygous; codon 317 CGC --> TGC and codon 142 GCT --> ACT), were selected for this study. The decrease in the number of IMPs with increase in their size was most marked in band 4.2 Komatsu, which was clinically most severe with no band 4.2 protein. In this regard, in band 4.2 Nippon, which showed moderate severity in clinical hematology with a nearly missing band 4.2 protein, increased sizing was less marked. The abnormalities in IMPs were the least in band 4.2 Shiga, which demonstrated compensated hemolysis with band 4.2 protein in a trace amount. The extent of the impairment of IMPs may be reflected by the total absence or the presence of band 4.2 protein even in a trace amount and/or by the specific site(s) of the mutation of the band 4.2 gene. Derangement of the cytoskeletal network was also observed in these three patients. It was most abnormal in band 4.2 Komatsu, and less so in band 4.2 Nippon and in band 4.2 Shiga. These results clearly indicate that 1) band 4.2 plays an important role not only in its binding to band 3 but also to the skeletal network (mostly to spectrins) vertically, and 2) its deficiency produces critical abnormality in maintenance of the structural and functional integrity of the integral proteins (such as band 3), as well as the cytoskeletal network.