Simple recipe for blocking ion channels

Domain-Specific Antibodies Reveal Differences in the Membrane Topologies of Apolipoprotein L1 in Serum and Podocytes

BACKGROUND

Circulating APOL1 lyses trypanosomes, protecting against human sleeping sickness. Two common African gene variants of APOL1, G1 and G2, protect against infection by species of trypanosomes that resist wild-type APOL1. At the same time, the protection predisposes humans to CKD, an elegant example of balanced polymorphism. However, the exact mechanism of APOL1-mediated podocyte damage is not clear, including APOL1's subcellular localization, topology, and whether the damage is related to trypanolysis.

METHODS

APOL1 topology in serum (HDL particles) and in kidney podocytes was mapped with flow cytometry, immunoprecipitation, and trypanolysis assays that tracked 170 APOL1 domain-specific monoclonal antibodies. APOL1 knockout podocytes confirmed antibody specificity.

RESULTS

APOL1 localizes to the surface of podocytes, with most of the pore-forming domain (PFD) and C terminus of the Serum Resistance Associated-interacting domain (SRA-ID), but not the membrane-addressing domain (MAD), being exposed. In contrast, differential trypanolytic blocking activity reveals that the MAD is exposed in serum APOL1, with less of the PFD accessible. Low pH did not detectably alter the gross topology of APOL1, as determined by antibody accessibility, in serum or on podocytes.

CONCLUSIONS

Our antibodies highlighted different conformations of native APOL1 topology in serum (HDL particles) and at the podocyte surface. Our findings support the surface ion channel model for APOL1 risk variant-mediated podocyte injury, as well as providing domain accessibility information for designing APOL1-targeted therapeutics.

TRPV4 and K(Ca) ion channels functionally couple as osmosensors in the paraventricular nucleus

BACKGROUND AND PURPOSE

Transient receptor potential vanilloid type 4 (TRPV4) and calcium-activated potassium channels (KCa ) mediate osmosensing in many tissues. Both TRPV4 and KCa channels are found in the paraventricular nucleus (PVN) of the hypothalamus, an area critical for sympathetic control of cardiovascular and renal function. Here, we have investigated whether TRPV4 channels functionally couple to KCa channels to mediate osmosensing in PVN parvocellular neurones and have characterized, pharmacologically, the subtype of KCa channel involved.

EXPERIMENTAL APPROACH

We investigated osmosensing roles for TRPV4 and KCa channels in parvocellular PVN neurones using cell-attached and whole-cell electrophysiology in mouse brain slices and rat isolated PVN neurons. Intracellular Ca(2+) was recorded using Fura-2AM. The system was modelled in the NEURON simulation environment.

KEY RESULTS

Hypotonic saline reduced action current frequency in hypothalamic slices; a response mimicked by TRPV4 channel agonists 4αPDD (1 μM) and GSK1016790A (100 nM), and blocked by inhibitors of either TRPV4 channels (RN1734 (5 μM) and HC067047 (300 nM) or the low-conductance calcium-activated potassium (SK) channel (UCL-1684 30 nM); iberiotoxin and TRAM-34 had no effect. Our model was compatible with coupling between TRPV4 and KCa channels, predicting the presence of positive and negative feedback loops. These predictions were verified using isolated PVN neurons. Both hypotonic challenge and 4αPDD increased intracellular Ca(2+) and UCL-1684 reduced the action of hypotonic challenge.

CONCLUSIONS AND IMPLICATIONS

There was functional coupling between TRPV4 and SK channels in parvocellular neurones. This mechanism contributes to osmosensing in the PVN and may provide a novel pharmacological target for the cardiovascular or renal systems.

Anti-KCNQ1 K⁺ channel autoantibodies increase IKs current and are associated with QT interval shortening in dilated cardiomyopathy

AIMS

Autoimmune-associated proarrhythmia in dilated cardiomyopathy (DCM) is poorly understood. Given the significance of KCNQ1 potassium channels in heart rhythm disorders, we hypothesized that circulating anti-KCNQ1 autoantibodies directly modulate cardiac electrophysiology in DCM patients. The purpose of this pilot study was to characterize ion channel autoantibodies in DCM targeting the cardiac repolarizing K(+) current, IKs, and the underlying KCNQ1 potassium channel.

METHODS AND RESULTS

One hundred and fifty DCM patients were screened for anti-KCNQ1 autoantibodies using an enzyme-linked immunosorbent assay. Autoantibodies targeting the extracellular pore domain of the KCNQ1 channel were detected in 6% of study patients. Seropositive individuals exhibited significantly shorter corrected QT intervals when compared with seronegative patients (371 ± 39.9 ms vs. 408 ± 47.9 ms; P = 0.036). There was no difference in clinical severity of heart failure between groups. The functional significance of anti-KCNQ1 antibodies was determined in human embryonic kidney 293 cells expressing KCNQ1/KCNE1 using the whole-cell patch clamp technique. IKs recordings demonstrated a 2.7-fold increase in mean current density on exposure to patients' sera containing anti-KCNQ1 antibodies in contrast to seronegative controls (8.74 ± 1.44 pA/pF vs. 3.26 ± 0.36 pA/pF; P = 0.003). IKs enhancement was not associated with increased KCNQ1 protein levels or altered cell surface expression of the channel.

CONCLUSION

Anti-KCNQ1 autoantibodies found in a subgroup of DCM patients are associated with QT interval shortening and increased IKs current.