The Gln4863Ala mutation within a putative, pore-lining trans-membrane helix of the cardiac ryanodine receptor channel alters both the kinetics of ryanoid interaction and the subsequent fractional conductance

Functional characterization of the cardiac ryanodine receptor pore-forming region

Ryanodine receptors are homotetrameric intracellular calcium release channels. The efficiency of these channels is underpinned by exceptional rates of cation translocation through the open channel and this is achieved at the expense of the high degree of selectivity characteristic of many other types of channel. Crystallization of prokaryotic potassium channels has provided insights into the structures and mechanisms responsible for ion selection and movement in these channels, however no equivalent structural detail is currently available for ryanodine receptors. Nevertheless both molecular modeling and cryo-electron microscopy have identified the probable pore-forming region (PFR) of the ryanodine receptor (RyR) and suggest that this region contains structural elements equivalent to those of the PFRs of potassium-selective channels. The aim of the current study was to establish if the isolated putative cardiac RyR (RyR2) PFR could form a functional ion channel. We have expressed and purified the RyR2 PFR and shown that function is retained following reconstitution into planar phospholipid bilayers. Our data provide the first direct experimental evidence to support the proposal that the conduction pathway of RyR2 is formed by structural elements equivalent to those of the potassium channel PFR.

Investigations of the contribution of a putative glycine hinge to ryanodine receptor channel gating

Ryanodine receptor channels (RyR) are key components of striated muscle excitation-contraction coupling, and alterations in their function underlie both inherited and acquired disease. A full understanding of the disease process will require a detailed knowledge of the mechanisms and structures involved in RyR function. Unfortunately, high-resolution structural data, such as exist for K⁺-selective channels, are not available for RyR. In the absence of these data, we have used modeling to identify similarities in the structural elements of K⁺ channel pore-forming regions and postulated equivalent regions of RyR. This has identified a sequence of residues in the cytosolic cavity-lining transmembrane helix of RyR (G⁴⁸⁶⁴LIIDA⁴⁸⁶⁹ in RyR2) analogous to the glycine hinge motif present in many K⁺ channels. Gating in these K⁺ channels can be disrupted by substitution of residues for the hinge glycine. We investigated the involvement of glycine 4864 in RyR2 gating by monitoring properties of recombinant human RyR2 channels in which this glycine is replaced by residues that alter gating in K⁺ channels. Our data demonstrate that introducing alanine at position 4864 produces no significant change in RyR2 function. In contrast, function is altered when glycine 4864 is replaced by either valine or proline, the former preventing channel opening and the latter modifying both ion translocation and gating. Our studies reveal novel information on the structural basis of RyR gating, identifying both similarities with, and differences from, K⁺ channels. Glycine 4864 is not absolutely required for channel gating, but some flexibility at this point in the cavity-lining transmembrane helix is necessary for normal RyR function.

Inositol 1,4,5-trisphosphate receptor regulates replication, differentiation, infectivity and virulence of the parasitic protist Trypanosoma cruzi

In animals, inositol 1,4,5-trisphosphate receptors (IP3 Rs) are ion channels that play a pivotal role in many biological processes by mediating Ca(2+) release from the endoplasmic reticulum. Here, we report the identification and characterization of a novel IP3 R in the parasitic protist, Trypanosoma cruzi, the pathogen responsible for Chagas disease. DT40 cells lacking endogenous IP3 R genes expressing T. cruzi IP3 R (TcIP3 R) exhibited IP3 -mediated Ca(2+) release from the ER, and demonstrated receptor binding to IP3 . TcIP3 R was expressed throughout the parasite life cycle but the expression level was much lower in bloodstream trypomastigotes than in intracellular amastigotes or epimastigotes. Disruption of two of the three TcIP3 R gene loci led to the death of the parasite, suggesting that IP3 R is essential for T. cruzi. Parasites expressing reduced or increased levels of TcIP3 R displayed defects in growth, transformation and infectivity, indicating that TcIP3 R is an important regulator of the parasite's life cycle. Furthermore, mice infected with T. cruzi expressing reduced levels of TcIP3 R exhibited a reduction of disease symptoms, indicating that TcIP3 R is an important virulence factor. Combined with the fact that the primary structure of TcIP3 R has low similarity to that of mammalian IP3 Rs, TcIP3 R is a promising drug target for Chagas disease.

Identification of intracellular and plasma membrane calcium channel homologues in pathogenic parasites

Ca²⁺ channels regulate many crucial processes within cells and their abnormal activity can be damaging to cell survival, suggesting that they might represent attractive therapeutic targets in pathogenic organisms. Parasitic diseases such as malaria, leishmaniasis, trypanosomiasis and schistosomiasis are responsible for millions of deaths each year worldwide. The genomes of many pathogenic parasites have recently been sequenced, opening the way for rational design of targeted therapies. We analyzed genomes of pathogenic protozoan parasites as well as the genome of Schistosoma mansoni, and show the existence within them of genes encoding homologues of mammalian intracellular Ca²⁺ release channels: inositol 1,4,5-trisphosphate receptors (IP₃Rs), ryanodine receptors (RyRs), two-pore Ca²⁺ channels (TPCs) and intracellular transient receptor potential (Trp) channels. The genomes of Trypanosoma, Leishmania and S. mansoni parasites encode IP₃R/RyR and Trp channel homologues, and that of S. mansoni additionally encodes a TPC homologue. In contrast, apicomplexan parasites lack genes encoding IP₃R/RyR homologues and possess only genes encoding TPC and Trp channel homologues (Toxoplasma gondii) or Trp channel homologues alone. The genomes of parasites also encode homologues of mammalian Ca²⁺influx channels, including voltage-gated Ca²⁺ channels and plasma membrane Trp channels. The genome of S. mansoni also encodes Orai Ca²⁺ channel and STIM Ca²⁺ sensor homologues, suggesting that store-operated Ca²⁺ entry may occur in this parasite. Many anti-parasitic agents alter parasite Ca²⁺ homeostasis and some are known modulators of mammalian Ca²⁺ channels, suggesting that parasite Ca²⁺ channel homologues might be the targets of some current anti-parasitic drugs. Differences between human and parasite Ca²⁺ channels suggest that pathogen-specific targeting of these channels may be an attractive therapeutic prospect.

Quantification of the effects of a ryanodine receptor channel mutation on interaction with a ryanoid

Understanding the nature of the interaction of the plant alkaloid ryanodine with its receptor channel (RyR) is important to aid interpretation of physiological studies and provide structure-function information about RyR. We present here the first quantitative description of the relative single-channel kinetic effects of a single-point mutation in RyR2. We exploit the well-characterized ryanoid 8beta-amino-9alpha-hydroxyryanodine that displays reversible kinetics with RyR2. We explicitly show that the effect of the Q4863A mutation is to increase the apparent dissociation constant by increasing the apparent dissociation rate of the ryanoid. The voltage-dependence of the interaction displays no change. We infer that Q4863 is not involved with the voltage-drop but is able to influence ryanoid-bound structural changes. We discuss structural mechanisms by which this mutation could affect ryanoid interaction.

The interaction of an impermeant cation with the sheep cardiac RyR channel alters ryanoid association

In previous studies, we have demonstrated that the interaction of ryanoids with the sarcoplasmic reticulum Ca(2+)-release channel [ryanodine receptor (RyR)] incorporated into planar lipid bilayers reduced the effectiveness of tetraethylammonium (TEA(+)) as a blocker of K(+) translocation (J Gen Physiol 117: 385-393, 2001). In the current study, we investigated both the effect of TEA(+) on [(3)H]ryanodine binding and the actions of this impermeant cation on the interaction of the reversible ryanoid 21-amino-9alpha-hydroxyryanodine with individual, voltage-clamped RyR channels. A dose-dependent inhibition of [(3)H]ryanodine binding was observed in the presence of TEA(+), suggesting that the cation and alkaloid compete for access to a common site of interaction. Single channel studies gave further insights into the mechanism of the competition between the two classes of ligands. TEA(+) decreases the association rate of 21-amino-9alpha-hydroxyryanodine with its receptor, whereas the dissociation rate of the ryanoid from the channel was unaffected. Our results demonstrate that TEA(+) inhibits both K(+) translocation through RyR, and ryanoid interaction at the high affinity ryanodine site on the channel. These actions involve binding of TEA(+) to different, but weakly interacting, sites in the RyR channel.