Mutations in PIEZO1 are the primary cause of hereditary xerocytosis, a clinically heterogeneous, dominantly inherited disorder of erythrocyte dehydration. We used next-generation sequencing-based techniques to identify mutations in individuals from 9 kindreds referred with suspected hereditary xerocytosis (HX) and/or undiagnosed congenital hemolytic anemia. Mutations were primarily found in the highly conserved, COOH-terminal pore-region domain. Several mutations were novel and demonstrated ethnic specificity. We characterized these mutations using genomic-, bioinformatic-, cell biology-, and physiology-based functional assays. For these studies, we created a novel, cell-based in vivo system for study of wild-type and variant PIEZO1 membrane protein expression, trafficking, and electrophysiology in a rigorous manner. Previous reports have indicated HX-associated PIEZO1 variants exhibit a partial gain-of-function phenotype with generation of mechanically activated currents that inactivate more slowly than wild type, indicating that increased cation permeability may lead to dehydration of PIEZO1-mutant HX erythrocytes. In addition to delayed channel inactivation, we found additional alterations in mutant PIEZO1 channel kinetics, differences in response to osmotic stress, and altered membrane protein trafficking, predicting variant alleles that worsen or ameliorate erythrocyte hydration. These results extend the genetic heterogeneity observed in HX and indicate that various pathophysiologic mechanisms contribute to the HX phenotype.

Key Points Mutations in the Gardos channel, encoded by the KCNN4 gene, were identified in individuals from 2 hereditary xerocytosis kindreds. These findings support recent data indicating the Gardos channel plays a role in normal erythrocyte volume homeostasis.

Plasmodium falciparum relies on anion channels activated in the erythrocyte membrane to ensure the transport of nutrients and waste products necessary for its replication and survival after invasion. The molecular identity of these anion channels, termed "new permeability pathways" is unknown, but their currents correspond to up-regulation of endogenous channels displaying complex gating and kinetics similar to those of ligand-gated channels. This report demonstrates that a peripheral-type benzodiazepine receptor, including the voltage dependent anion channel, is present in the human erythrocyte membrane. This receptor mediates the maxi-anion currents previously described in the erythrocyte membrane. Ligands that block this peripheral-type benzodiazepine receptor reduce membrane transport and conductance in P falciparum-infected erythrocytes. These ligands also inhibit in vitro intraerythrocytic growth of P falciparum. These data support the hypothesis that dormant peripheral-type benzodiazepine receptors become the "new permeability pathways" in infected erythrocytes after upregulation by P falciparum. These channels are obvious targets for selective inhibition in anti-malarial therapies, as well as potential routes for drug delivery in pharmacologic applications. (Blood. 2011; 118(8):2305-2312) IntroductionThe most severe form of malaria in humans is caused by parasite Plasmodium falciparum, infecting 225 million people and causing 781 000 deaths in 2009 (World Health Organization, 2010). Erythrocyte invasion by P falciparum provides the parasite access to a plentiful source of nutrients in a locale that is largely shielded from host immune defenses. After invasion, the invading parasite uses a variety of strategies to adapt to the intraerythrocytic environment. To ensure the transport of nutrients and waste products necessary for its replication and survival, P falciparum relies on broad specificity anion channels activated in the erythrocyte membrane after invasion. 1 Initially, this transport was attributed to "new" permeability pathways (NPPs) 2 exported by the parasite to the host membrane. 3 However, later studies revealed that the current is because of up-regulation of endogenous channels 4 and that the diversity of anion channel activities recorded in these studies correspond to different kinetic modalities of a unique type of maxi-anion channel. 5 This channel displays complex gating and kinetics similar to those of ligand-gated channels. 5 Anions are transported through the human erythrocyte membrane by a 2-component system: a large electroneutral exchanger mediated by band 3 and a 4 orders of magnitude smaller electrogenic component estimated at approximately 10 S/cm 2 corresponding presumably to a small number of channels. 6 Remarkably, the molecular identification and characterization of this conductive pathway has not yet been achieved. Neither genomic nor proteomic studies have provided meaningful clues to the composition of this pathway. 7 Considering the small amount of protein a few hundred c...

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