Characterization of the ryanodine receptor/channel of invertebrate muscle

Elongation and Contraction of Scallop Sarcoplasmic Reticulum (SR): ATP Stabilizes Ca2+-ATPase Crystalline Array Elongation of SR Vesicles

The Ca²⁺-ATPase is an integral transmembrane Ca²⁺ pump of the sarcoplasmic reticulum (SR). Crystallization of the cytoplasmic surface ATPase molecules of isolated scallop SR vesicles was studied at various calcium concentrations by negative stain electron microscopy. In the absence of ATP, round SR vesicles displaying an assembly of small crystalline patches of ATPase molecules were observed at 18 µM [Ca²⁺]. These partly transformed into tightly elongated vesicles containing ATPase crystalline arrays at low [Ca²⁺] (≤1.3 µM). The arrays were classified as ‘’tetramer’’, “two-rail” (like a railroad) and ‘’monomer’’. Their crystallinity was low, and they were unstable. In the presence of ATP (5 mM) at a low [Ca²⁺] of ~0.002 µM, “two-rail” arrays of high crystallinity appeared more frequently in the tightly elongated vesicles and the distinct tetramer arrays disappeared. During prolonged (~2.5 h) incubation, ATP was consumed and tetramer arrays reappeared. A specific ATPase inhibitor, thapsigargin, prevented both crystal formation and vesicle elongation in the presence of ATP. Together with the second part of this study, these data suggest that the ATPase forms tetramer units and longer tetramer crystalline arrays to elongate SR vesicles, and that the arrays transform into more stable “two-rail” forms in the presence of ATP at low [Ca²⁺].

Opisthorchis felineus genes differentially expressed under praziquantel shed light on the nature of tegument disruption and indicate the adaptive role of cGMP-dependent protein kinase

Opisthorchis felineus is a trematode flatworm that parasitises mammals, including humans, and is mainly spread throughout Eastern Europe and Western Siberia. The main drug used in treatment of opisthorchiasis and other trematode and cestode infestations is praziquantel (PZQ). We provide a possible explanation of PZQ-mediated tegument disruption. The idea is that the nature of tegument disruption is related to failure of surface renovation due to insufficiency of microtubule transport of vesicles. This insufficiency arises from microtubule destabilisation, which in the medium term leads to the decrease in tubulins alpha, beta and dynein mRNA amounts and deficiency of the corresponding proteins. We also found the upregulation of cGMP-dependent protein kinase gene, and we concluded that its protein product helped to overcome the effect of praziquantel and might be a promising target for combined anthelmintic therapy with PZQ. We concluded that function of saposin-like protein 2 (SAP2) is unlikely associated with membrane fusion, and SAP2 is probably able to bind some type of hydrophobic compounds including praziquantel.

Ryanodine receptors: structure, expression, molecular details, and function in calcium release

Ryanodine receptors (RyRs) are located in the sarcoplasmic/endoplasmic reticulum membrane and are responsible for the release of Ca(2+) from intracellular stores during excitation-contraction coupling in both cardiac and skeletal muscle. RyRs are the largest known ion channels (> 2MDa) and exist as three mammalian isoforms (RyR 1-3), all of which are homotetrameric proteins that interact with and are regulated by phosphorylation, redox modifications, and a variety of small proteins and ions. Most RyR channel modulators interact with the large cytoplasmic domain whereas the carboxy-terminal portion of the protein forms the ion-conducting pore. Mutations in RyR2 are associated with human disorders such as catecholaminergic polymorphic ventricular tachycardia whereas mutations in RyR1 underlie diseases such as central core disease and malignant hyperthermia. This chapter examines the current concepts of the structure, function and regulation of RyRs and assesses the current state of understanding of their roles in associated disorders.

From action potential to contraction: neural control and excitation-contraction coupling in larval muscles of Drosophila

The neuromuscular system of Drosophila melanogaster has been studied for many years for its relative simplicity and because of the genetic and molecular versatilities. Three main types of striated muscles are present in this dipteran: fibrillar muscles, tubular muscles and supercontractile muscles. The visceral muscles in adult flies and the body wall segmental muscles in embryos and larvae belong to the group of supercontractile muscles. Larval body wall muscles have been the object of detailed studies as a model for neuromuscular junction function but have received much less attention with respect to their mechanical properties and to the control of contraction. In this review we wish to assess available information on the physiology of the Drosophila larval muscular system. Our aim is to establish whether this system has the requisites to be considered a good model in which to perform a functional characterization of Drosophila genes, with a known muscular expression, as well as Drosophila homologs of human genes, the dysfunction of which, is known to be associated with human hereditary muscle pathologies.

Effects of Mg2+ on Ca2+ release from sarcoplasmic reticulum of skeletal muscle fibres from yabby (crustacean) and rat

1. The role of myoplasmic [Mg2+] on Ca2+ release from the sarcoplasmic reticulum (SR) was examined in the two major types of crustacean muscle fibres, the tonic, long sarcomere fibres and the phasic, short sarcomere fibres of the fresh water decapod crustacean Cherax destructor (yabby) and in the fast-twitch rat muscle fibres using the mechanically skinned muscle fibre preparation. 2. A robust Ca2+-induced Ca2+-release (CICR) mechanism was present in both long and short sarcomere fibres and 1 mM Mg2+ exerted a strong inhibitory action on the SR Ca2+ release in both fibre types. 3. The SR displayed different properties with respect to Ca2+ loading in the long and the short sarcomere fibres and marked functional differences were identified with respect to Mg2+ inhibition between the two crustacean fibre types. Thus, in long sarcomere fibres, the submaximally loaded SR was able to release Ca2+ when [Mg2+] was lowered from 1 to 0.01 mM in the presence of 8 mM ATPtotal and in the virtual absence of Ca2+ (< 5 nM) even when the CICR was suppressed. In contrast, negligible Ca2+ was released from the submaximally loaded SR of short sarcomere yabby fibres when [Mg2+] was lowered from 1 to 0.01 mM under the same conditions as for the long sarcomere fibres. Nevertheless, the rate of SR Ca2+ release in short sarcomere fibres increased markedly when [Mg2+] was lowered in the presence of [Ca2+] approaching the normal resting levels (50-100 nM). 4. Rat fibres were able to release SR Ca2+ at a faster rate than the long sarcomere yabby fibres when [Mg2+] was lowered from 1 to 0. 01 mM in the virtual absence of Ca2+ but, unlike with yabby fibres, the net rate of Ca2+ release was actually increased for conditions that were considerably less favourable to CICR. 5. In summary, it is concluded that crustacean skeletal muscles have more that one functional type of Ca2+-release channels, that these channels display properties that are intermediate between those of mammalian skeletal and cardiac isoforms, that the inhibition exerted by Mg2+ at rest on the crustacean SR Ca2+-release channels must be removed during excitation-contraction coupling and that, unlike in crustacean fibres, CICR cannot play the major role in the activation of SR Ca2+-release channels in the rat skeletal muscle.

Reversible block of the calcium release channel/ryanodine receptor by protamine, a heparin antidote

Channel activity of the calcium release channel from skeletal muscle, ryanodine receptor type 1, was measured in the presence and absence of protamine sulfate on the cytoplasmic side of the channel. Single-channel activity was measured after incorporating channels into planar lipid bilayers. Optimally and suboptimally calcium-activated calcium release channels were inactivated by the application of protamine to the cytoplasmic side of the channel. Recovery of channel activity was not observed while protamine was present. The addition of protamine bound to agarose beads did not change channel activity, implying that the mechanism of action involves an interaction with the ryanodine receptor rather than changes in the bulk calcium concentration of the medium. The block of channel activity by protamine could be reversed either by removal by perfusion with buffer or by the addition of heparin to the cytoplasmic side of the channel. Microinjection of protamine into differentiated C(2)C(12) mouse muscle cells prevented caffeine-induced intracellular calcium release. The results suggest that protamine acts on the ryanodine receptor in a similar but opposite manner from heparin and that protamine can be used as a potent, reversible inhibitor of ryanodine receptor activity.

Evidence for novel caffeine and Ca2+ binding sites on the lobster skeletal ryanodine receptor

1. The effects of Ca2+, ATP and caffeine on the gating of lobster skeletal muscle ryanodine receptors (RyR) was investigated after reconstitution of the channels into planar phospholipid bilayers and by using [3H]-ryanodine binding studies. 2. The single channel studies reveal that the EC50 (60 microM) for activation of the lobster skeletal RyR by Ca2+ as the sole ligand is higher than for any other isoform of RyR studied. 3. Inactivation of the channel by Ca2+ (EC50 = 1 mM) occurs at concentrations slightly higher than those required to inactivate mammalian skeletal RyR (RyR1) but lower than those required to inactivate mammalian cardiac RyR (RyR2). 4. Lifetime analysis demonstrates that cytosolic Ca2+, as the sole activating ligand, cannot fully open the lobster skeletal RyR (maximum Po approximately 0.2). The mechanism for the increase in open probability (Po) is an increase in both the frequency and the duration of the open events. 5. ATP is a very effective activator of the lobster RyR and can almost fully open the channel in the presence of activating cytosolic [Ca2+]. In the presence of 700 microM Ca2+, 1 mM ATP increased Po to approximately 0.8. 6. Caffeine, often used as a tool to identify the presence of RyR channels, is relatively ineffective and cannot increase Po above the level that can be attained with Ca2+ alone. 7. The results reveal that caffeine increases Po by a different mechanism to that of cytosolic Ca2+ demonstrating that the mechanism for channel activation by caffeine is not 'sensitization' to cytosolic Ca2+. 8. By studying the mechanisms involved in the activation of the lobster RyR we have demonstrated that the channel responds in a unique manner to Ca2+ and to caffeine. The results strongly indicate that these ligand binding sites on the channel are different to those on mammalian isoforms of RyR.