Single channel function of inositol 1,4,5-trisphosphate receptor type-1 and -2 isoform domain-swap chimeras

Membrane protein reconstitution for functional and structural studies

Membrane proteins are involved in various critical biological processes, and studying membrane proteins represents a major challenge in protein biochemistry. As shown by both structural and functional studies, the membrane environment plays an essential role for membrane proteins. In vitro studies are reliant on the successful reconstitution of membrane proteins. This review describes the interaction between detergents and lipids that aids the understanding of the reconstitution processes. Then the techniques of detergent removal and a few useful techniques to refine the formed proteoliposomes are reviewed. Finally the applications of reconstitution techniques to study membrane proteins involved in Ca(2+) signaling are summarized.

Analysis of IP3 receptors in and out of cells

BACKGROUND

Inositol 1,4,5-trisphosphate receptors (IP3R) are expressed in almost all animal cells. Three mammalian genes encode closely related IP3R subunits, which assemble into homo- or hetero-tetramers to form intracellular Ca2+ channels.

SCOPE OF THE REVIEW

In this brief review, we first consider a variety of complementary methods that allow the links between IP3 binding and channel gating to be defined. How does IP3 binding to the IP3-binding core in each IP3R subunit cause opening of a cation-selective pore formed by residues towards the C-terminal? We then describe methods that allow IP3, Ca2+ signals and IP3R mobility to be examined in intact cells. A final section briefly considers genetic analyses of IP3R signalling.

MAJOR CONCLUSIONS

All IP3R are regulated by both IP3 and Ca2+. This allows them to initiate and regeneratively propagate intracellular Ca2+ signals. The elementary Ca2+ release events evoked by IP3 in intact cells are mediated by very small numbers of active IP3R and the Ca2+-mediated interactions between them. The spatial organization of these Ca2+ signals and their stochastic dependence on so few IP3Rs highlight the need for methods that allow the spatial organization of IP3R signalling to be addressed with single-molecule resolution.

GENERAL SIGNIFICANCE

A variety of complementary methods provide insight into the structural basis of IP3R activation and the contributions of IP3-evoked Ca2+ signals to cellular physiology. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

IP3 receptors: some lessons from DT40 cells

Inositol-1,4,5-trisphosphate receptors (IP3Rs) are intracellular Ca2+ channels that are regulated by IP3 and Ca2+ and are modulated by many additional signals. These properties allow them to initiate and, via Ca2+-induced Ca2+ release, regeneratively propagate Ca2+ signals evoked by receptors that stimulate formation of IP3. The ubiquitous expression of IP3R highlights their importance, but it also presents problems when attempting to resolve the behavior of defined IP3R. DT40 cells are a pre-B-lymphocyte cell line in which high rates of homologous recombination afford unrivalled opportunities to disrupt endogenous genes. DT40-knockout cells with both alleles of each of the three IP3R genes disrupted provide the only null-background for analysis of homogenous recombinant IP3R. We review the properties of DT40 cells and consider three areas where they have contributed to understanding IP3R behavior. Patch-clamp recording from the nuclear envelope and Ca2+ release from intracellular stores loaded with a low-affinity Ca2+ indicator address the mechanisms leading to activation of IP(3)R. We show that IP3 causes intracellular IP3R to cluster and re-tune their responses to IP3 and Ca2+, better equipping them to mediate regenerative Ca2+ signals. Finally, we show that DT40 cells reliably count very few IP3R into the plasma membrane, where they mediate about half the Ca2+ entry evoked by the B-cell antigen receptor.

IP3 receptor-dependent Ca2+ release modulates excitation-contraction coupling in rabbit ventricular myocytes

Inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R)-dependent Ca(2+) signaling exerts positive inotropic, but also arrhythmogenic, effects on excitation-contraction coupling (ECC) in the atrial myocardium. The role of IP(3)R-dependent sarcoplasmic reticulum (SR) Ca(2+) release in ECC in the ventricular myocardium remains controversial. Here we investigated the role of this signaling pathway during ECC in isolated rabbit ventricular myocytes. Immunoblotting of proteins from ventricular myocytes showed expression of both type 2 and type 3 IP(3)R at levels approximately 3.5-fold less than in atrial myocytes. In permeabilized myocytes, direct application of IP(3) (10 microM) produced a transient 21% increase in the frequency of Ca(2+) sparks (P < 0.05). This increase was accompanied by a 13% decrease in spark amplitude (P < 0.05) and a 7% decrease in SR Ca(2+) load (P < 0.05) and was inhibited by IP(3)R antagonists 2-aminoethoxydiphenylborate (2-APB; 20 microM) and heparin (0.5 mg/ml). In intact myocytes endothelin-1 (100 nM) was used to stimulate IP(3) production and caused a 38% (P < 0.05) increase in the amplitude of action potential-induced (0.5 Hz, field stimulation) Ca(2+) transients. This effect was abolished by the IP(3)R antagonist 2-APB (2 microM) or by using adenoviral expression of an IP(3) affinity trap that buffers cellular IP(3). Together, these data suggest that in rabbit ventricular myocytes IP(3)R-dependent Ca(2+) release has positive inotropic effects on ECC by facilitating Ca(2+) release through ryanodine receptor clusters.

Inositol trisphosphate receptor Ca2+ release channels

The inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are a family of Ca2+ release channels localized predominately in the endoplasmic reticulum of all cell types. They function to release Ca2+ into the cytoplasm in response to InsP3 produced by diverse stimuli, generating complex local and global Ca2+ signals that regulate numerous cell physiological processes ranging from gene transcription to secretion to learning and memory. The InsP3R is a calcium-selective cation channel whose gating is regulated not only by InsP3, but by other ligands as well, in particular cytoplasmic Ca2+. Over the last decade, detailed quantitative studies of InsP3R channel function and its regulation by ligands and interacting proteins have provided new insights into a remarkable richness of channel regulation and of the structural aspects that underlie signal transduction and permeation. Here, we focus on these developments and review and synthesize the literature regarding the structure and single-channel properties of the InsP3R.

Calcium regulation of inositol 1,4,5-trisphosphate receptors

Ca2+ exerts both a stimulatory and inhibitory effect on type-I IP3R channel activity. However, the structural determinants of Ca2+ sensing in IP3Rs are not fully understood. Previous studies by others have identified eight domains of the type-I IP3R that bind 45Ca2+ when expressed as GST-fusion proteins. We have mutated six highly conserved acidic residues within the second of these domains (aa378-450) in the full-length IP3R and measured the Ca2+ regulation of IP3-mediated Ca2+ release in COS-7 cells. 45Ca2+ flux assays measured with a maximal [IP3] (1 microM) indicate that one of the mutants retained a Ca2+ sensitivity that was not significantly different from control (E411Q), three of the mutants show an enhanced Ca2+ inhibition (D426N, E428Q and E439Q) and two of the mutants were relatively insensitive to Ca2+ inhibition (D442N and D444N). IP3 dose-response relationships indicated that the sensitivity to Ca2+ inhibition and affinity for IP3 were correlated for three of the constructs. Other mutants with enhanced IP3 sensitivity (e.g. R441Q and a type-II/I IP3R chimera) were also less sensitive to Ca2+ inhibition. We conclude that the acidic residues within the aa378-450 segment are unlikely to represent a single functional Ca2+ binding domain and do not contribute to Ca2+ activation of the receptor. The different effects of the mutations may be related to their location within two clusters of acidic residues identified in the crystal structure of the ligand-binding domain [I. Bosanac, J.R. Alattia, T.K. Mal, et al., Structure of the inositol 1,4,5-trisphosphate receptor binding core in complex with its ligand, Nature 420 (2002) 696-700]. The data support the view that all IP3R isoforms may display a range of Ca2+ sensitivities that are determined by multiple sites within the protein and markedly influenced by the affinity of the receptor for IP3.

Modulation of mammalian inositol 1,4,5-trisphosphate receptor isoforms by calcium: a role of calcium sensor region

In the accompanying article, we compared main functional properties of the three mammalian inositol 1,4,5-trisphosphate receptors (InsP3R) isoforms. In this article we focused on modulation of mammalian InsP3R isoforms by cytosolic Ca2+. We found that: 1), when recorded in the presence of 2 microM InsP3 and 0.5 mM ATP all three mammalian InsP3R isoforms display bell-shaped Ca2+ dependence in physiological range of Ca2+ concentrations (pCa 8-5); 2), in the same experimental conditions InsP3R3 is most sensitive to modulation by Ca2+ (peak at 107 nM Ca2+), followed by InsP3R2 (peak at 154 nM Ca2+), and then by InsP3R1 (peak at 257 nM Ca2+); 3), increase in ATP concentration to 5 mM had no significant effect of Ca2+ dependence of InsP3R1 and InsP3R2; 4), increase in ATP concentration to 5 mM converted Ca2+ dependence of InsP3R3 from "narrow" shape to "square" shape; 5), ATP-induced change in the shape of InsP3R3 Ca2+ dependence was mainly due to an >200-fold reduction in the apparent affinity of the Ca2+-inhibitory site; 6), the apparent Ca2+ affinity of the Ca2+ sensor region (Cas) determined in biochemical experiments is equal to 0.23 microM Ca2+ for RT1-Cas, 0.16 microM Ca2+ for RT2-Cas, and 0.10 microM Ca2+ for RT3-Cas; and 7), Ca2+ sensitivity of InsP3R1 and InsP3R3 isoforms recorded in the presence of 2 microM InsP3 and 0.5 mM ATP or 2 microM InsP3 and 5 mM ATP can be exchanged by swapping their Cas regions. Obtained results provide novel information about functional properties of mammalian InsP3R isoforms and support the importance of the Ca2+ sensor region (Cas) in determining the sensitivity of InsP3R isoforms to modulation by Ca2+.