Regulation of the ryanodine receptor calcium release channel: a molecular complex system

Voltage-induced Ca2+ release by ryanodine receptors causes neuropeptide secretion from nerve terminals

Depolarisation-secretion coupling is assumed to be dependent only on extracellular calcium ([Ca²⁺ ]_o ). Ryanodine receptor (RyR)-sensitive stores in hypothalamic neurohypophysial system (HNS) terminals produce sparks of intracellular calcium ([Ca²⁺ ]_i ) that are voltage-dependent. We hypothesised that voltage-elicited increases in intraterminal calcium are crucial for neuropeptide secretion from presynaptic terminals, whether from influx through voltage-gated calcium channels and/or from such voltage-sensitive ryanodine-mediated calcium stores. Increases in [Ca²⁺ ]_i upon depolarisation in the presence of voltage-gated calcium channel blockers, or in the absence of [Ca²⁺ ]_o , still give rise to neuropeptide secretion from HNS terminals. Even in 0 [Ca²⁺ ]_o , there was nonetheless an increase in capacitance suggesting exocytosis upon depolarisation. This was blocked by antagonist concentrations of ryanodine, as was peptide secretion elicited by high K⁺ in 0 [Ca²⁺ ]_o . Furthermore, such depolarisations lead to increases in [Ca²⁺ ]_i . Pre-incubation with BAPTA-AM resulted in > 50% inhibition of peptide secretion elicited by high K⁺ in 0 [Ca²⁺ ]_o . Nifedipine but not nicardipine inhibited both the high K⁺ response for neuropeptide secretion and intraterminal calcium, suggesting the involvement of CaV1.1 type channels as sensors in voltage-induced calcium release. Importantly, RyR antagonists also modulate neuropeptide release under normal physiological conditions. In conclusion, our results indicate that depolarisation-induced neuropeptide secretion is present in the absence of external calcium, and calcium release from ryanodine-sensitive internal stores is a significant physiological contributor to neuropeptide secretion from HNS terminals.

Novel diuretic targets

As the molecular revolution continues to inform a deeper understanding of disease mechanisms and pathways, there exist unprecedented opportunities for translating discoveries at the bench into novel therapies for improving human health. Despite the availability of several different classes of antihypertensive medications, only about half of the 67 million Americans with hypertension manage their blood pressure appropriately. A broader selection of structurally diverse antihypertensive drugs acting through different mechanisms would provide clinicians with greater flexibility in developing effective treatment regimens for an increasingly diverse and aging patient population. An emerging body of physiological, genetic, and pharmacological evidence has implicated several renal ion-transport proteins, or regulators thereof, as novel, yet clinically unexploited, diuretic targets. These include the renal outer medullary potassium channel, ROMK (Kir1.1), Kir4.1/5.1 potassium channels, ClC-Ka/b chloride channels, UTA/B urea transporters, the chloride/bicarbonate exchanger pendrin, and the STE20/SPS1-related proline/alanine-rich kinase (SPAK). The molecular pharmacology of these putative targets is poorly developed or lacking altogether; however, recent efforts by a few academic and pharmaceutical laboratories have begun to lessen this critical barrier. Here, we review the evidence in support of the aforementioned proteins as novel diuretic targets and highlight examples where progress toward developing small-molecule pharmacology has been made.

A designed inhibitor of a CLC antiporter blocks function through a unique binding mode

The lack of small-molecule inhibitors for anion-selective transporters and channels has impeded our understanding of the complex mechanisms that underlie ion passage. The ubiquitous CLC "Chloride Channel" family represents a unique target for biophysical and biochemical studies because its distinctive protein fold supports both passive chloride channels and secondary-active chloride-proton transporters. Here, we describe the synthesis and characterization of a specific small-molecule inhibitor directed against a CLC antiporter (ClC-ec1). This compound, 4,4'-octanamidostilbene-2,2'-disulfonate (OADS), inhibits ClC-ec1 with low micromolar affinity and has no specific effect on a CLC channel (ClC-1). Inhibition of ClC-ec1 occurs by binding to two distinct intracellular sites. The location of these sites and the lipid dependence of inhibition suggest potential mechanisms of action. This compound will empower research to elucidate differences between antiporter and channel mechanisms and to develop treatments for CLC-mediated disorders.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Design of dantrolene-derived probes for radioisotope-free photoaffinity labeling of proteins involved in the physiological Ca2+ release from sarcoplasmic reticulum of skeletal muscle

Bifunctional dantrolene derivatives have been synthesized as probes for radioisotope-free photoaffinity labeling with the aim of elucidating the molecular mechanism of skeletal muscle contraction. GIF-0430 and GIF-0665 are aromatic azido-functionalized derivatives that were designed to selectively inhibit physiological Ca2+ release (PCR) from sarcoplasmic reticulum (SR) in mouse skeletal muscle without a strong effect on Ca2+-induced Ca2+ release (CICR). These photoaffinity probes consist of either an azidomethyl or an ethynyl group, respectively, which could function as a tag for introduction of an optional detectable marker unit by an appropriate chemoselective ligation method after the photo-cross-linking operation. Actually, the former probe worked to photolabel its target proteins specifically as confirmed by subsequent fluorescent visualization.

Dantrolene analogues revisited: general synthesis and specific functions capable of discriminating two kinds of Ca2+ release from sarcoplasmic reticulum of mouse skeletal muscle

The general synthesis of dantrolene analogues with various substituents on its phenyl ring has been developed via palladium-catalyzed cross-coupling reactions, the Stille or Suzuki reaction, as the key step. The effects of synthesized analogues have been evaluated by two kinds of Ca(2+) release modes from sarcoplasmic reticulum (SR) of mouse skeletal muscle fibers based on: (1) the measurement of twitch contraction caused by the physiological Ca(2+) release (PCR) of intact skeletal muscle and (2) the rate of Ca(2+)-induced Ca(2+) release (CICR) in saponin-treated skinned muscle fibers. Although dantrolene, a lead compound, inhibits both twitch contraction and CICR, some structurally modified analogues exhibit one or the other of these effects. The methoxy congener, GIF-0185, potently inhibits the twitch contraction without affecting the CICR, while GIF-0166 and GIF-0248, the ortho-nitro regioisomer and ortho, ortho-dinitro substituted analogues, respectively, doubly potentiate the CICR exclusively.

[(125)I]-N-[(3-azido-5-iodo)benzyl]dantrolene and [(125)I]-N-[[3-iodo-5-(3-trifluoromethyl-3H-diazirin-3-yl)]benzyl]dantrolene: photoaffinity probes specific for the physiological Ca(2+) release from sarcoplasmic reticulum of skeletal muscle

In order to capture and identify key molecules that regulate the release of Ca(2+) from the sarcoplasmic reticulum (SR) of skeletal muscle, we designed specific photoaffinity probes based on the structural modification of dantrolene. Thus, GIF-0082 and GIF-0276 possessing azido- and trifluoromethyldiazirinyl-benzyl groups, respectively, at the hydantoin moiety were found to have a highly selective inhibitory effect on physiological Ca(2+) release (PCR) without affecting Ca(2+)-induced Ca(2+) release (CICR). Successful realization of the sharp discrimination between PCR and CICR has led to the creation of [(125)I]GIF-0082 and [(125)I]GIF-0276, which were synthesized by substituting a stannyl group with (125)I in the corresponding phenylstannane precursors.