Calcium Influx: Beyond ‘Current’ Biology

A novel, cell-surface protein essential for Ca2+ release-activated Ca2+ (CRAC) channel function has been identified through independent genome-wide screens. This huge advance will enable molecular dissection of the CRAC channel complex, moving the field beyond Icrac signature to structure

IP3 Receptor Activity Is Differentially Regulated in Endoplasmic Reticulum Subdomains during Oocyte Maturation

SummaryFertilization competency results from hormone-induced remodeling of oocytes into eggs. The signaling pathways that effect this change exemplify bistability, where brief hormone exposure irrevocably switches cell fate. In Xenopus, changes in Ca2+ signaling epitomize such remodeling: The reversible Ca2+ signaling phenotype of oocytes rapidly adapts to support irreversible propagation of the fertilization Ca2+ wave. Here, we simultaneously resolved IP3 receptor (IP3R) activity with endoplasmic reticulum (ER) structure to optically dissect the functional architecture of the Ca2+ release apparatus underpinning this reorganization. We show that changes in Ca2+ signaling correlate with IP3R redistribution from specialized ER substructures called annulate lamellae (AL), where Ca2+ release activity is attenuated, into IP3R-replete patches in the cortical ER of eggs that support the fertilization Ca2+ wave. These data show: first, that IP3R sensitivity is regulated with high spatial acuity even between contiguous ER regions; and second, that drastic reorganization of Ca2+ signaling dynamics can be driven by subcellular redistribution in the absence of changes in channel number or molecular or familial Ca2+ channel diversity. Finally, these results define a novel role for AL in Ca2+ signaling. Because AL are prevalent in other scenarios of rapid cell division, further studies of their impact on Ca2+ signaling are warranted

Timing in Cellular Ca2+ Signaling

SummaryCalcium (Ca2+) signals are generated across a broad time range. Kinetic considerations impact how information is processed to encode and decode Ca2+ signals, the choreography of responses that ensure specific and efficient signaling and the overall temporal amplification such that ephemeral Ca2+ signals have lasting physiological value. The reciprocal importance of timing for Ca2+ signaling, and Ca2+ signaling for timing is exemplified by the altered kinetic profiles of Ca2+ signals in certain diseases and the likely role of basal Ca2+ fluctuations in the perception of time itself

Cooperative activation of IP3 receptors by sequential binding of IP3 and Ca2+ safeguards against spontaneous activity

AbstractBackground: Ca2+ waves allow effective delivery of intracellular Ca2+ signals to cytosolic targets. Propagation of these regenerative Ca2+ signals probably results from the activation of intracellular Ca2+ channels by the increase in cytosolic [Ca2+] that follows the opening of these channels. Such positive feedback is potentially explosive. Mechanisms that limit the spontaneous opening of intracellular Ca2+ channels are therefore likely to have evolved in parallel with the mechanism of Ca2+-induced Ca2+ release.Results: Maximal rates of 45Ca2+ efflux from permeabilised hepatocytes superfused with medium in which the [Ca2+] was clamped were cooperatively stimulated by inositol 1,4,5-trisphosphate (IP3). A minimal interval of ∼400 msec between IP3 addition and the peak rate of Ca2+ mobilisation indicate that channel opening does not immediately follow binding of IP3. Although the absolute latency of Ca2+ release was unaffected by further increasing the IP3 concentration, it was reduced by increased [Ca2+].Conclusions: We propose that the closed conformation of the IP3 receptor is very stable and therefore minimally susceptible to spontaneous activation; at least three (probably four) IP3 molecules may be required to provide enough binding energy to drive the receptor into a stable open conformation. We suggest that a further defence from noise is provided by an extreme form of coincidence detection. Binding of IP3 to each of its four receptor subunits unmasks a site to which Ca2+ must bind before the channel can open. As IP3 binding may also initiate receptor inactivation, there may be only a narrow temporal window during which each receptor subunit must bind both of its agonists if the channel is to open rather than inactivate

Localization and socialization: experimental insights into the functional architecture of IP₃ receptors

Inositol 1,4,5-trisphosphate (IP₃)-evoked Ca²⁺ signals display great spatiotemporal malleability. This malleability depends on diversity in both the cellular organization and in situ functionality of IP₃ receptors (IP₃Rs) that regulate Ca²⁺ release from the endoplasmic reticulum (ER). Recent experimental data imply that these considerations are not independent, such that—as with other ion channels—the local organization of IP₃Rs impacts their functionality, and reciprocally IP₃R activity impacts their organization within native ER membranes. Here, we (i) review experimental data that lead to our understanding of the “functional architecture” of IP₃Rs within the ER, (ii) propose an updated terminology to span the organizational hierarchy of IP₃Rs observed in intact cells, and (iii) speculate on the physiological significance of IP₃R socialization in Ca²⁺ dynamics, and consequently the emerging need for modeling studies to move beyond gridded, planar, and static simulations of IP₃R clustering even over short experimental timescales.Centro Regional de Estudios Genómico

Modulation of Elementary Calcium Release Mediates a Transition from Puffs to Waves in an IP3R Cluster Model

The oscillating concentration of intracellular calcium is one of the most important examples for collective dynamics in cell biology. Localized releases of calcium through clusters of inositol 1,4,5-trisphosphate receptor channels constitute elementary signals called calcium puffs. Coupling by diffusing calcium leads to global releases and waves, but the exact mechanism of inter- cluster coupling and triggering of waves is unknown. To elucidate the relation of puffs and waves, we here model a cluster of IP3R channels using a gating scheme with variable non-equilibrium IP3 binding. Hybrid stochastic and deterministic simulations show that puffs are not stereotyped events of constant duration but are sensitive to stimulation strength and residual calcium. For increasing IP3 concentration, the release events become modulated at a timescale of minutes, with repetitive wave-like releases interspersed with several puffs. This modulation is consistent with experimental observations we present, including refractoriness and increase of puff frequency during the inter-wave interval. Our results suggest that waves are established by a random but time-modulated appearance of sustained release events, which have a high potential to trigger and synchronize activity throughout the cell

Praziquantel activates a native cation current in Schistosoma mansoni

IntroductionPraziquantel (PZQ), an anthelmintic drug discovered in the 1970s, is still used to treat schistosomiasis and various other infections caused by parasitic flatworms. PZQ causes a triad of phenotypic effects on schistosome worms – rapid depolarization, muscle contraction, and damage throughout the worm tegument. The molecular target mediating these effects has been intimated as a Ca2+-permeable ion channel, but native currents evoked by PZQ have not been reported in any schistosome cell type. The properties of the endogenous PZQ activated conductance therefore remain unknown. MethodsHere, invasive electrophysiology was used to probe for responses to PZQ from different locales in a living schistosome worm.Results and discussionNo direct response was seen in tegument-derived vesicles, or from the sub-tegumental muscle layer despite the presence of voltage-operated currents. However, PZQ rapidly triggered a sustained, non-selective cation current in recordings from neuronal tissue, targeting both the anterior ganglion and the main longitudinal nerve cord. The biophysical signature of this PZQ-evoked current resolved at single channel resolution matched that of a transient receptor potential ion channel named TRPMPZQ, recently proposed as the molecular target of PZQ. The endogenous PZQ-evoked current was also inhibited by a validated TRPMPZQ antagonist. PZQ therefore is a neuroactive anthelmintic, causing a sustained depolarization through ion channels with the characteristics of TRPMPZQ