Depletion “skraps” and dynamic buffering inside the cellular calcium store

Abstract

Ca 2؉ signals, produced by Ca 2؉ release from cellular stores, switch metabolic responses inside cells. In muscle, Ca 2؉ sparks locally exhibit the rapid start and termination of the cell-wide signal. By imaging Ca 2؉ inside the store using shifted excitation and emission ratioing of fluorescence, a surprising observation was made: Depletion during sparks or voltage-induced cell-wide release occurs too late, continuing to progress even after the Ca 2؉ release channels have closed. This finding indicates that Ca 2؉ is released from a ''proximate'' compartment functionally in between store lumen and cytosol. The presence of a proximate compartment also explains a paradoxical surge in intrastore Ca 2؉ , which was recorded upon stimulation of prolonged, cell-wide Ca 2؉ release. An intrastore surge upon induction of Ca 2؉ release was first reported in subcellular store fractions, where its source was traced to the store buffer, calsequestrin. The present results update the evolving concept, largely due to N. Ikemoto and C. Kang, of calsequestrin as a dynamic store. Given the strategic location and reduction of dimensionality of Ca 2؉ -adsorbing linear polymers of calsequestrin, they could deliver Ca 2؉ to the open release channels more efficiently than the luminal store solution, thus constituting the proximate compartment. When store depletion becomes widespread, the polymers would collapse to increase store [Ca 2؉ ] and sustain the concentration gradient that drives release flux.calcium signaling ͉ calcium sparks ͉ excitation-contraction coupling ͉ sarcoplasmic reticulum ͉ skeletal muscle R apid changes in intracellular cytosolic [Ca 2ϩ ] are required for signaling functions in many cell types (1). These changes are achieved via Ca 2ϩ release through channels, ryanodine receptors (RyRs), which must open and close quickly. To increase its speed, gating of RyRs relies on effects of the permeant ion, including channel opening by elevated cytosolic [Ca 2ϩ ] (2). In muscle, the desirable fast kinetic features are already present in its elementary signaling events, Ca 2ϩ sparks (3), which involve the nearly simultaneous opening (4) of a number of channels (5), followed by their synchronized closing (4). Thus, this gating does not follow the usual Markovian rules for channels that evolve independently but requires timekeeping and synchronization (6). In cardiac muscle, depletion of sarcoplasmic reticulum (SR) Ca 2ϩ is the likely timer of channel closing, and the substantial depletion that follows the cardiac beat (7) was imaged as ''blinks'' associated with Ca 2ϩ sparks (8). The sensor that translates depletion into channel closing appears to be the main intra-SR buffer, calsequestrin (CSQ) (9).By contrast, in skeletal muscle, depletion associated with a twitch is only 8-15% (10). This low rate of depletion reflects a SR with larger terminal cisternae containing higher concentrations of a CSQ of greater binding capacity, thus constituting a much greater calcium reservoir. Despite the greater store, sparks of skeletal mus...

Calsequestrin depolymerizes when calcium is depleted in the sarcoplasmic reticulum of working muscle

“…Intra-SR calcium transients were first reported in vesicular SR fractions (35) and studied in detail in the rat (36). With simultaneous measurements of FRAP rate and [Ca 2+ ] SR , we now show that they coexist with an increase in Casq1 mobility, confirming that they are associated with and, in all likelihood, due to changes in the polymeric network of Casq1, as proposed by Launikonis et al (37).…”

IP3 Receptors: Toward Understanding Their Activation

“…Persuasive evidence suggests that Ca 2þ release by RyR may be terminated before Ca 2þ stores are entirely depleted because luminal Ca 2þ is required to maintain RyR activity (Györke and Györke 1998;Launikonis et al 2006;Jiang et al 2008), possibly via its interaction with calsequestrin, a luminal high-capacity Ca 2þ -binding protein (Launikonis et al 2006;Terentyev et al 2006). A similar scheme has been proposed to account for two features of IP 3 -evoked Ca 2þ release: the initiation of Ca 2þ release after the quiescent interspike interval during repetitive Ca 2þ spikes (Berridge 2007) and quantal Ca 2þ release via IP 3 R. The latter describes the situation wherein unidirectional Ca 2þ efflux from intracellular stores terminates before the stores have fully emptied after stimulation with submaximally effective concentrations of IP 3 without loss of their ability to respond to a further increase in IP 3 concentration (Muallem et al 1989;Meyer and Stryer 1990;Taylor and Potter 1990;Oldershaw et al 1991;Bootman et al 1992;Brown et al 1992;Combettes et al 1992;Ferris et al 1992;Hirota et al 1995).…”

Upregulation of store-operated Ca2+ entry in dystrophic mdx mouse muscle

“…While this preparation loses some cytoplasmic factors following skinning and immersion in an internal solution, it still, for example, retains functional EC coupling that is not different from that of intact fibers (27,36). In this study we compare the kinetics of SOCE activation and deactivation in adult fibers from wild-type (WT) and mdx mouse muscle during depleting SR Ca 2ϩ release.…”