The ladder-shaped polyether toxin gambierol anchors the gating machinery of Kv3.1 channels in the resting state

Voltage-gated potassium (Kv) and sodium (Nav) channels are key determinants of cellular excitability and serve as targets of neurotoxins. Most marine ciguatoxins potentiate Nav channels and cause ciguatera seafood poisoning. Several ciguatoxins have also been shown to affect Kv channels, and we showed previously that the ladder-shaped polyether toxin gambierol is a potent Kv channel inhibitor. Most likely, gambierol acts via a lipid-exposed binding site, located outside the K(+) permeation pathway. However, the mechanism by which gambierol inhibits Kv channels remained unknown. Using gating and ionic current analysis to investigate how gambierol affected S6 gate opening and voltage-sensing domain (VSD) movements, we show that the resting (closed) channel conformation forms the high-affinity state for gambierol. The voltage dependence of activation was shifted by >120 mV in the depolarizing direction, precluding channel opening in the physiological voltage range. The (early) transitions between the resting and the open state were monitored with gating currents, and provided evidence that strong depolarizations allowed VSD movement up to the activated-not-open state. However, for transition to the fully open (ion-conducting) state, the toxin first needed to dissociate. These dissociation kinetics were markedly accelerated in the activated-not-open state, presumably because this state displayed a much lower affinity for gambierol. A tetrameric concatemer with only one high-affinity binding site still displayed high toxin sensitivity, suggesting that interaction with a single binding site prevented the concerted step required for channel opening. We propose a mechanism whereby gambierol anchors the channel’s gating machinery in the resting state, requiring more work from the VSD to open the channel. This mechanism is quite different from the action of classical gating modifier peptides (e.g., hanatoxin). Therefore, polyether toxins open new opportunities in structure–function relationship studies in Kv channels and in drug design to modulate channel function

Functional correlates of somatostatin receptor 2 over-expression in the retina of mice with genetic deletion of somatostatin receptor 1

Somatostatin-14 (SRIF) and its receptors (sst(1-5)) are found in the mammalian retina. However, scarce information is available on the role of the somatostatinergic system in retinal physiology. We have recently used gene-knockout technology to gain insights into the function of sst(1) and sst(2) receptors in the mouse retina. The sst(1) receptor localizes to SRIF-containing amacrine cells, whereas the sst(2) receptor localizes to several retinal cell populations including rod bipolar cells (RBCs). Molecular data indicate that, in retinas with deletion of the sst(1) receptor (sst(1) KO), sst(2) receptors become overexpressed in concomitance with an increased level of retinal SRIF. To test whether this up-regulation of sst(2) receptors correlates with altered sst(2) receptor physiology, we studied the effect of sst(2) receptor activation on potassium current (I(K)) in isolated RBCs and glutamate release in retina explants. Both I(K) and glutamate release are known to be negatively modulated by sst(2) receptors in the mammalian retina. We used octreotide, a SRIF analogue, to activate selectively sst(2) receptors. Patch-clamp recordings from isolated RBCs indicated that the sst(2) receptor-mediated inhibition of I(K) was significantly larger in sst(1) KO than in control retinas. In addition, HPLC measurements of glutamate release in sst(1) KO retinal explants demonstrated that the sst(2) receptor-mediated inhibition of K(+)-evoked glutamate release was also significantly larger than in control retinas. As a whole, these findings indicate that the overexpression of sst(2) receptors in sst(1) KO retinas can be correlated to an enhanced function of sst(2) receptors. The level of expression of sst(2) receptors may therefore represent a key step in the regulation of sst(2) receptor-mediated responses, at least in the retina

Leydig Cell Tumour in a 46,XX Child with Congenital Adrenal Hyperplasia due to 21-Hydroxylase Deficiency

Case Report: A 10-year-old male was referred to our institution due to short stature and bilateral cryptorchidism and reported pubic hair development and acne since the age of 4 years. Laboratory and molecular genetic tests indicated congenital adrenal hyperplasia due to 21-hydroxylase deficiency. After treatment with prednisone, adrenal hormones normalised but testosterone remained elevated. Magnetic resonance imaging of the abdomen due to cryptorchidism revealed uterus and adnexal attachments, a prostate and poorly defined nodules on the iliac chains. Upon exploratory laparotomy, a hysterectomy, bilateral oophorectomy and resection of a peri-adnexal nodular lesion on the patient's right side were performed. Histopathology of the nodule mass was compatible with a Leydig cell tumour with a low proliferation rate according to Ki67. Copyright (c) 2013 S. Karger AG, Basel79317918