Decreased prion protein expression in human peripheral blood leucocytes from patients with paroxysmal nocturnal haemoglobinuria

Prion protein E219K polymorphism: from the discovery of the KANNO blood group to interventions for human prion disease

KANNO is a new human blood group that was recently discovered. The KANNO antigen shares the PRNP gene with the prion protein and the prion protein E219K polymorphism determines the presence or absence of the KANNO antigen and the development of anti-KANNO alloantibodies. These alloantibodies specifically react with prion proteins, which serve as substrates for conversion into pathological isoforms in some prion diseases and may serve as effective targets for resisting prion infection. These findings establish a potential link between the KANNO blood group and human prion disease via the prion protein E219K polymorphism. We reviewed the interesting correlation between the human PRNP gene's E219K polymorphism and the prion proteins it expresses, as well as human red blood cell antigens. Based on the immune serological principles of human blood cells, the prion protein E219K polymorphism may serve as a foundation for earlier molecular diagnosis and future drug development for prion diseases.

Physiological role of the cellular prion protein

The prion protein (PrP) plays a key role in the pathogenesis of prion diseases. However, the normal function of the protein remains unclear. The cellular isoform (PrP(C)) is expressed most abundantly in the brain, but has also been detected in other non-neuronal tissues as diverse as lymphoid cells, lung, heart, kidney, gastrointestinal tract, muscle, and mammary glands. Cell biological studies of PrP contribute to our understanding of PrP(C) function. Like other membrane proteins, PrP(C) is post-translationally processed in the endoplasmic reticulum and Golgi on its way to the cell surface after synthesis. Cell surface PrP(C) constitutively cycles between the plasma membrane and early endosomes via a clathrin-dependent mechanism, a pathway consistent with a suggested role for PrP(C) in cellular trafficking of copper ions. Although PrP(-/-) mice have been reported to have only minor alterations in immune function, PrP(C) is up-regulated in T cell activation and may be expressed at higher levels by specialized classes of lymphocytes. Furthermore, antibody cross-linking of surface PrP(C) modulates T cell activation and leads to rearrangements of lipid raft constituents and increased phosphorylation of signaling proteins. These findings appear to indicate an important but, as yet, ill-defined role in T cell function. Recent work has suggested that PrP(C) is required for self-renewal of haematopoietic stem cells. PrP(C) is highly expressed in the central nervous system, and since this is the major site of prion pathology, most interest has focused on defining the role of PrP(C) in neurones. Although PrP(-/-) mice have a grossly normal neurological phenotype, even when neuronal PrP(C) is knocked out postnatally, they do have subtle abnormalities in synaptic transmission, hippocampal morphology, circadian rhythms, and cognition and seizure threshold. Other postulated neuronal roles for PrP(C) include copper-binding, as an anti- and conversely, pro-apoptotic protein, as a signaling molecule, and in supporting neuronal morphology and adhesion. The prion protein may also function as a metal binding protein such as copper, yielding cellular antioxidant capacity suggesting a role in the oxidative stress homeostasis. Finally, recent observations on the role of PrP(C) in long-term memory open a challenging field.

CD34+ cells from paroxysmal nocturnal hemoglobinuria (PNH) patients are deficient in surface expression of cellular prion protein (PrPc)

Cellular prion protein (PrP(c)) is a glycosylphosphatidylinositol (GPI)-anchored protein (GPI-AP) constitutively expressed by neurons but also in hematopoietic cells. In trasmissible spongiform encephalopathies, the protease-resistant form of prion (PrP (s c)) converts the host PrP(c) into the pathologic form. We have investigated PrP(c) expression in hematopoietic cells from paroxysmal nocturnal hemoglobinuria (PNH). In this disease, due to somatic mutations in PIG-A gene, biosynthesis of the (GPI)-anchor is impaired and affected cells lack membrane expression of all GPI-AP. Normal and PNH hematopoietic progenitors and paired wild-type (WT) and PIG-A mutant cell lines were used for analysis of intracellular and surface PrP(c) expression using flow cytometry and Western blot.By flow cytometry, PrP(c) was constitutively present on normal CD34(+) cells, including more immature CD38(dim) cells, as well as hematopoietic cell lines. Similar results were obtained in purified CD34(+). Phospholipase C treatment confirmed that PrP(c) was expressed on the membrane via the GPI-anchor. In PNH patients, GPI-AP-deficient CD34(+) cells lacked PrP(c) membrane expression. PIG-A-mutated cell lines (Jurkat, K562, C(EBV), A(EBV)), in contrast to their normal counterparts, did not express surface PrP(c). However, we detected intracellular PrP(c) at approximately equivalent levels in both normal and PIG-A-mutated cells using intracellular flow cytometry and Western blotting. Cells and cell lines with PNH phenotype together with their normal counterparts may be a suitable system to explore the function of membrane PrP(c) in the hematopoietic system. Conversely, PrP(c) is a good model to elucidate the fate of GPI-AP in PIG-A-deficient cells.

Activated platelets of patients with paroxysmal nocturnal hemoglobinuria express cellular prion protein

Cellular prion protein (PrPc) is a glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein that contains a putative membrane-spanning section. Patients with paroxysmal nocturnal hemoglobinuria (PNH) lack GPI proteins on the surface of somatically mutated hematopoietic stem cell and its progeny. Platelet expression of PrPc was studied in 8 PNH patients. Resting PNH (CD55(-)) platelets were devoid of surface PrPc, but activation of platelets resulted in the surface expression of PrPc. Expressed PrPc was detected by 2 monoclonal antibodies (mAbs) against the N-terminal part of the molecule but not by mAb 6H4, which binds at the C-terminus beyond the membrane-spanning section. However, 6H4 detected PrPc on Western blots of PNH platelets, demonstrating that the lack of 6H4 binding was not caused by PrPc truncation. Our results indicate that in the absence of GPI anchor, PrPc can be expressed intracellularly and up-regulated on the platelet membrane, likely in a transmembrane form with the C-terminal part of the molecule inserted into the cytoplasm.

All-trans retinoic acid down-regulates prion protein expression independently of granulocyte maturation

The cellular prion protein (PrPc) is a sialoglycoprotein involved in the pathogenesis of prion diseases. It has been identified at the plasma membrane of several cell types. All-trans retinoic acid (ATRA) is known to induce differentiation of human leukemia cell lines in vitro. PrPc messenger ribonucleic acid (mRNA) and protein are down-regulated upon ATRA-induced differentiation of HL60 cells. In this report, we have investigated the regulation of PrPc mRNA and protein expression during ATRA-treatment of maturation-sensitive (NB4) and -resistant (NB4-R1 and NB4-R2) cell lines. In ATRA-induced maturation of NB4 cells, down-regulation of PrPc mRNA and protein were observed. We also show that down-regulation of PrPc mRNA is dependent on protein synthesis. Moreover, the same down-regulation of prion protein by ATRA was observed at the surface of maturation-resistant, ATRA-responsive NB4-R1 cells. In contrast, the maturation-resistant and ATRA-unresponsive NB4-R2 subline showed no variation in membrane prion protein expression. These results demonstrate a dissociation between the regulation of prion protein expression by ATRA and the process of granulocyte maturation. We propose that retinoids should be investigated further as a preventive strategy to slow down prion disease progression.