Ablation of the Kell/Xk complex alters erythrocyte divalent cation homeostasis

Abstract

XK is a putative transporter of unknown function that is ubiquitously expressed and linked through disulfide bonds to Kell protein, an endothelin-3 (ET-3)-converting enzyme. We generated three knockout (KO) mice that lacked either Xk, Kell or both proteins and characterized erythrocyte cation levels, transport and hematological parameters. Absence of Xk or Kell was accompanied by changes in erythrocyte K+, Mg2+, Na+ and Ca2+ transport that were associated with changes in mean cellular volume and corpuscular hemoglobin concentration mean. Baseline Ca2+-ATPase activity was undetected in erythrocytes from all three mouse types but was restored upon pre-incubation with ET-3. Consistent with these alterations in Ca2+ handling, we observed increased Gardos channel activity in Kel and Xk KO mice. In addition Kel deletion was associated with increased Mg2+ permeability while Xk deletion blocked Na/Mg exchanger activity. Our results provide evidence that cellular divalent cation regulation is functionally coupled to the Kell/XK system in erythrocytes and loss of this complex may contribute to acanthocytosis formation in McLeod syndrome.

Exposure of Phosphatidylserine by Xk-related Protein Family Members during Apoptosis

“…Rivera et al (47) recently reported that XK (Xkr1) regulates cell volume by transporting divalent cations. Of the 10-member TMEM16 family, five members function as Ca 2ϩ -dependent phospholipid scramblases, whereas two members carry Cl Ϫ channel activity (12,48,49).…”

The human Kell blood group binds the erythroid 4.1R protein: new insights into the 4.1R-dependent red cell membrane complex

“…In addition, the Kell protein shares a consensus sequence with the large family of zinc endopeptidases and has endothelin-3 converting enzyme activity of type II membrane glycoproteins. Gene disruption in mice provided evidence that cellular divalent cation regulation is functionally coupled to the Kell/XK system in erythrocytes and loss of this complex might contribute to the acanthocytosis seen in McLeod syndrome (Rivera , et al 2013). A rare phenotype termed Kell null (Ko) is characterized by the absence of Kell protein and Kell antigens from the red cell membrane and diminished amounts of XK protein (Khamlichi , et al 1995, Redman , et al 1999).…”

Stepwise partitioning of Xp21: a profiling method for XK deletions causative of the McLeod syndrome

“…[10][11][12] In the RBC membrane, XK exists as heterodimer together with the membrane Kell protein explaining the weakened expression of all Kell antigens in Kx-deficient individuals. [13][14][15] Today, the pathophysiologic link between Kx negativity and the onset of MLS is still unclear 1 although Rivera and colleagues 16 showed that deficiency of XK and XK/Kell proteins led to altered transmembrane transport of divalent cations. 17 Absent Kx RBC antigen is pathognomonic for patients with MLS and therefore represents the diagnostic marker of highest specificity.…”