Anion and Cation Permeability of the Mouse TMEM16F Calcium-Activated Channel

TMEM16F is involved in several physiological processes, such as blood coagulation, bone development and virus infections. This protein acts both as a Ca2+-dependent phospholipid scramblase and a Ca2+-activated ion channel but several studies have reported conflicting results about the ion selectivity of the TMEM16F-mediated current. Here, we have performed a detailed side-by-side comparison of the ion selectivity of TMEM16F using the whole-cell and inside-out excised patch configurations to directly compare the results. In inside-out configuration, Ca2+-dependent activation was fast and the TMEM16F-mediated current was activated in a few milliseconds, while in whole-cell recordings full activation required several minutes. We determined the relative permeability between Na+ and Cl¯ (PNa/PCl) using the dilution method in both configurations. The TMEM16F-mediated current was highly nonselective, but there were differences depending on the configuration of the recordings. In whole-cell recordings, PNa/PCl was approximately 0.5, indicating a slight preference for Cl¯ permeation. In contrast, in inside-out experiments the TMEM16F channel showed a higher permeability for Na+ with PNa/PCl reaching 3.7. Our results demonstrate that the time dependence of Ca2+ activation and the ion selectivity of TMEM16F depend on the recording configuration.

A Grammastola spatulata mechanotoxin-4 (GsMTx4)-sensitive cation channel mediates increased cation permeability in human hereditary spherocytosis of multiple genetic etiologies

Not available.

The TRPM7 Channel in the Nervous and Cardiovascular Systems

: Transient receptor potential melastatin 7 (TRPM7), along with the closely related TRPM6, are unique channels that have dual operations: cation permeability and kinase activity. In contrast to the limited tissue distribution of TRPM6, TRPM7 is widely expressed among tissues and is therefore implicated in a variety of cellular functions physiologically and pathophysiologically. The discovery of TRPM7’s unique structure imparting dual ion channel and kinase activities shed light onto novel and peculiar biological functions, such as Mg2+ homeostasis, cellular Ca2+ flickering, and even intranuclear transcriptional regulation by a cleaved kinase domain translocated to nuclei. Interestingly, at a higher level, TRPM7 participates in several biological processes in the nervous and cardiovascular systems, in which excitatory responses in neurons and cardiomyocytes are critical for their function. Here, we review the roles of TRPM7 in cells involved in the nervous and cardiovascular systems and discuss its potential as a future therapeutic target.

The Role of Eryptosis in the Pathogenesis of Renal Anemia: Insights From Basic Research and Mathematical Modeling

Red blood cells (RBC) are the most abundant cells in the blood. Despite powerful defense systems against chemical and mechanical stressors, their life span is limited to about 120 days in healthy humans and further shortened in patients with kidney failure. Changes in the cell membrane potential and cation permeability trigger a cascade of events that lead to exposure of phosphatidylserine on the outer leaflet of the RBC membrane. The translocation of phosphatidylserine is an important step in a process that eventually results in eryptosis, the programmed death of an RBC. The regulation of eryptosis is complex and involves several cellular pathways, such as the regulation of non-selective cation channels. Increased cytosolic calcium concentration results in scramblase and floppase activation, exposing phosphatidylserine on the cell surface, leading to early clearance of RBCs from the circulation by phagocytic cells. While eryptosis is physiologically meaningful to recycle iron and other RBC constituents in healthy subjects, it is augmented under pathological conditions, such as kidney failure. In chronic kidney disease (CKD) patients, the number of eryptotic RBC is significantly increased, resulting in a shortened RBC life span that further compounds renal anemia. In CKD patients, uremic toxins, oxidative stress, hypoxemia, and inflammation contribute to the increased eryptosis rate. Eryptosis may have an impact on renal anemia, and depending on the degree of shortened RBC life span, the administration of erythropoiesis-stimulating agents is often insufficient to attain desired hemoglobin target levels. The goal of this review is to indicate the importance of eryptosis as a process closely related to life span reduction, aggravating renal anemia.

Combinatorial expression of claudins in the proximal renal tubule and its functional consequences

The proximal renal tubule (PT) is characterized by a highly conductive paracellular pathway, which contributes to a significant amount of solute and water reabsorption by the kidney. Claudins are tight junction proteins that, in part, determine the paracellular permeability of epithelia. In the present study, we determined the expression pattern of the major PT claudins. We found that claudin-2 and claudin-10 are coexpressed throughout the PT, whereas claudin-3 is coexpressed with claudin-2 predominantly in the proximal straight tubule. Additionally, claudin-2 and claudin-3 are expressed separately within mutually exclusive populations of descending thin limbs. We developed a novel double-inducible Madin-Darby canine kidney I cell model to characterize in vitro the functional effect of coexpression of PT claudins. In keeping with previous studies, we found that claudin-2 alone primarily increased cation (Na+ and Ca2+) permeability, whereas claudin-10a alone increased anion (Cl−) permeability. Coexpression of claudin-2 and claudin-10a together led to a weak physical interaction between the isoforms and the formation of a monolayer with high conductance but neutral charge selectivity. Claudin-3 expression had a negligible effect on all measures of cell permeability, whether expressed alone or together with claudin-2. In cells coexpressing a claudin-2 mutant, S68C, together with claudin-10a, inhibition of cation permeability through the claudin-2 pore with a thiol-reactive pore blocker did not block anion permeation through claudin-10a. We conclude that claudin-2 and claudin-10a form independent paracellular cation- and anion-selective channels that function in parallel.

Inherited hemolytic anemia: a possessive beginner’s guide

Significant advances have been made in diagnosis and clinical management of inherited red cell membrane disorders that result in hemolytic anemia. Membrane structural defects lead to hereditary spherocytosis (HS) and hereditary elliptocytosis (HE), whereas altered membrane transport function accounts for hereditary xerocytosis (HX) and hereditary overhydrated stomatocytosis (OHS). The degrees of membrane loss and resultant increases in cell sphericity determine the severity of anemia in HS and HE, and splenectomy leads to amelioration of anemia by increasing the circulatory red cell life span. Alterations in cell volume as a result of disordered membrane cation permeability account for reduced life span red cells in HX and OHS. Importantly, splenectomy is not beneficial in these 2 membrane transport disorders and is not recommended because it is ineffective and may lead to an increased risk of life-threatening thrombosis. Rational approaches are now available for the diagnosis and management of these inherited red cell disorders, and these will be discussed in this review.

Pannexin1: a multifunction and multiconductance and/or permeability membrane channel

Of the three pannexins in vertebrate proteomes, pannexin1 (Panx1) is the only one well characterized, and it is generally accepted that Panx1 functions as an ATP release channel for signaling to other cells. However, the ATP permeability of the channel is only observed with certain stimuli, including low oxygen, mechanical stress, and elevated extracellular potassium ion concentration. Otherwise, the Panx1 channel is selective for chloride ions and exhibits no ATP permeability when stimulated simply by depolarization to positive potentials. A third, irreversible activation of Panx1 follows cleavage of carboxyterminal amino acids by caspase 3. The selectivity/permeability properties of the caspase cleaved channel are unclear as it reportedly has features of both channel conformations. Here we describe the biophysical properties of the channel formed by the truncation mutant Panx1Δ378, which is identical to the caspase-cleaved protein. Consistent with previous findings for the caspase-activated channel, the Panx1Δ378 channel was constitutively active. However, like the voltage-gated channel, the Panx1Δ378 channel had high chloride selectivity, lacked cation permeability, and did not mediate ATP release unless stimulated by extracellular potassium ions. Thus, the caspase-cleaved Panx1 channel should be impermeable to ATP, contrary to previous claims.