Genetic disorders of the red cell membrane☆

Alpha hemolysin (HlyA) is a hemolytic and cytotoxic protein secreted by uropathogenic strains of E. coli. The role of glycophorins (GPs) as putative receptors for HlyA binding to red blood cells (RBCs) has been debated.

Experiments using anti-GPA/GPB antibodies and a GPA-specific epitope nanobody to block HlyA-GP binding on hRBCs, showed no effect on hemolytic activity. Similarly, the hemolysis induced by HlyA remained unaffected when hRBCs from a GPA^null/GPB^null variant were used. Surface Plasmon Resonance experiments revealed similar values of the dissociation constant between GPA and either HlyA, ProHlyA (inactive protoxin), HlyAΔ914-936 (mutant of HlyA lacking the binding domain to GPA) or human serum albumin, indicating that the binding between the proteins and GPA is not specific.

Although far Western blot followed by mass spectroscopy analyses suggested that HlyA interacts with Band 3 and spectrins, hemolytic experiments on spectrin-depleted hRBCs and spherocytes, indicated these proteins do not mediate the hemolytic process.

Our results unequivocally demonstrate that neither glycophorins, nor Band 3 and spectrins mediate the cytotoxic activity of HlyA on hRBCs, thereby challenging the HlyA-receptor hypothesis. This finding holds significant relevance for the design of anti-toxin therapeutic strategies, particularly in light of the growing antibiotic resistance exhibited by bacteria.

The significant risks associated with total splenectomy have led to interest in the use of partial splenectomy as an alternative surgical therapy for children who have congenital hemolytic anemia. Partial splenectomy is designed to remove enough spleen to gain desired hematologic outcomes while preserving splenic immune function. Although preliminary data demonstrate successful laboratory and clinical outcomes after partial splenectomy in various congenital hemolytic anemias, conclusive data comparing the efficacy of partial splenectomy to total splenectomy are not reported. Based on preliminary data, a definitive clinical trial of partial splenectomy in children who have severe congenital hemolytic anemia may be warranted.

Partial splenectomy is an alternative to total splenectomy for the treatment of congenital hemolytic anemias (CHAs) in children, although the feasibility of this technique in the setting of massive splenomegaly is unknown. This study was designed to evaluate the safety and efficacy of partial splenectomy in children with CHAs and massive splenomegaly. This retrospective study examined 29 children with CHAs who underwent partial splenectomy. Children were divided into 2 groups based on splenic size: 8 children had splenic volumes greater than 500 mL, whereas 21 children had splenic volumes less than 500 mL. Outcome variables included perioperative complications, transfusion requirements, hematocrits, reticulocyte counts, bilirubin levels, splenic sequestration, and splenic regrowth. All 29 children underwent successful partial splenectomy with 0.02 to 10 years of follow-up. After partial splenectomy, children overall had decreased transfusion requirements, increased hematocrits, decreased bilirubin levels, decreased reticulocyte counts, and elimination of splenic sequestration. Children with massive splenomegaly had similar outcomes compared with children without massive splenomegaly. Long-term complications included 3 mild infections, 4 cases of gallstones requiring cholecystectomy, and 1 child who required completion splenectomy. Partial splenectomy is a safe, effective, and technically feasible option for children with various CHAs, even in the setting of massive splenomegaly.

In red blood cells, protein 4.1 (4.1R) is an 80-kDa protein that stabilizes the spectrin-actin network and anchors it to the plasma membrane. The picture is more complex in nucleated cells, in which many 4.1R isoforms, varying in size and intracellular location, have been identified. To contribute to the characterization of signals involved in differential intracellular localization of 4.1R, we have analyzed the role the exon 5-encoded sequence plays in 4.1R distribution. We show that exon 5 encodes a leucine-rich sequence that shares key features with nuclear export signals (NESs). This sequence adopts the topology employed for NESs of other proteins and conserves two hydrophobic residues that are shown to be critical for NES function. A 4.1R isoform expressing the leucine-rich sequence binds to the export receptor CRM1 in a RanGTP-dependent fashion, whereas this does not occur in a mutant whose two conserved hydrophobic residues are substituted. These two residues are also essential for 4.1R intracellular distribution, because the 4.1R protein containing the leucine-rich sequence localizes in the cytoplasm, whereas the mutant protein predominantly accumulates in the nucleus. We hypothesize that the leucine-rich sequence in 4.1R controls distribution and concomitantly function of a specific set of 4.1R isoforms.

Red blood cell protein 4.1 (4.1R) is an 80-kDa protein that stabilizes the spectrin-actin network and anchors it to the plasma membrane. To contribute to the characterization of functional roles and partners of specific nonerythroid 4.1R isoforms, we analyzed 4.1R in human T cells and found that endogenous 4.1R was distributed to the microtubule network. Transfection experiments of T cell 4.1R cDNAs in conjunction with confocal microscopy analysis revealed the colocalization of exogenous 4.1R isoforms with the tubulin skeleton. Biochemical analyses using Taxol®(paclitaxel)-polymerized microtubules from stably transfected T cells confirmed the association of the exogenous 4.1R proteins with microtubules. Consistent with this, endogenous 4.1R immunoreactive proteins were also detected in the microtubule-containing fraction.In vitro binding assays using glutathioneS-transferase-4.1R fusion proteins showed that a constitutive domain of the 4.1R molecule, one that is therefore present in all 4.1R isoforms, is responsible for the association with tubulin. A 22-amino acid sequence comprised in this domain and containing heptad repeats of leucine residues was essential for tubulin binding. Furthermore, ectopic expression of 4.1R in COS-7 cells provoked microtubule disorganization. Our results suggest an involvement of 4.1R in interphase microtubule architecture and support the hypothesis that some 4.1R functional activities are cell type-regulated.

A complex family of 4.1R isoforms has been identified in non-erythroid tissues. In this study we characterized the exonic composition of brain 4.1R-10-kDa orspectrin/actin binding (SAB) domain and identified the minimal sequences required to stimulate fodrin/F-actin association. Adult rat brain expresses predominantly 4.1R mRNAs that carry an extended SAB, consisting of the alternative exons 14/15/16 and part of the constitutive exon 17. Exon 16 along with sequences carried by exon 17 is necessary and sufficient to induce formation of fodrin-actin-4.1R ternary complexes. The ability of the respective SAB domains of 4.1 homologs to sediment fodrin/actin was also investigated. 4.1G-SAB stimulates association of fodrin/actin, although with an ∼2-fold reduced efficiency compared with 4.1R-10-kDa, whereas 4.1N and 4.1B do not. Sequencing of the corresponding domains revealed that 4.1G-SAB carries a cassette that shares significant homology with 4.1R exon 16, whereas the respective sequence is divergent in 4.1N and absent from brain 4.1B. An ∼150-kDa 4.1R and an ∼160-kDa 4.1G isoforms are present in PC12 lysates that occur in vivo in a supramolecular complex with fodrin and F-actin. Moreover, proteins 4.1R and 4.1G are distributed underneath the plasma membrane in PC12 cells. Collectively, these observations suggest that brain 4.1R and 4.1G may modulate the membrane mechanical properties of neuronal cells by promoting fodrin/actin association.