Imbalance in cerebral protein homeostasis: Effects on memory consolidation.

The long-standing hypothesis that memory consolidation is dependent upon de novo protein synthesis is based primarily on the amnestic effects of systemic administration of protein synthesis inhibitors (PSIs). Early experiments on mice showed that PSIs produced interference with memory consolidation that was dependent on the doses of PSIs, on the interval between drug injection and training, and, importantly, on the degree and duration of protein synthesis inhibition in the brain. Surprisingly, there is a conspicuous lack of information regarding the relationship between the duration of protein synthesis inhibition produced by PSIs and memory consolidation in the rat, one of the species most widely used to study memory processes. We found that, in the male rat, a single injection of cycloheximide, a commonly used PSI, produced a significant imbalance in protein homeostasis: an early inhibition of protein synthesis that lasted for at least one hour, followed by hyperproduction of proteins that lasted three days. We evaluated memory consolidation of inhibitory avoidance trained with either low or high intensity of foot-shock at the peaks of protein synthesis inhibition and protein hyperproduction. We found that, independent of the moment of training, the low-foot-shock groups showed amnesia, while the high-foot-shock groups displayed optimal memory performance. These results indicate that memory consolidation of relatively weak training is impaired by the inhibition or hyperproduction of protein synthesis, and that intense training overcomes this dysregulation of protein homeostasis allowing for memory formation probably through non-genomic mechanisms.

Mouse corticospinal system comprises different functional neuronal ensembles depending on their hodology.

BACKGROUND: Movement performance depends on the synaptic interactions generated by coherent parallel sensorimotor cortical outputs to different downstream targets. The major outputs of the neocortex to subcortical structures are driven by pyramidal tract neurons (PTNs) located in layer 5B. One of the main targets of PTNs is the spinal cord through the corticospinal (CS) system, which is formed by a complex collection of distinct CS circuits. However, little is known about intracortical synaptic interactions that originate CS commands and how different populations of CS neurons are functionally organized. To further understand the functional organization of the CS system, we analyzed the activity of unambiguously identified CS neurons projecting to different zones of the same spinal cord segment using two-photon calcium imaging and retrograde neuronal tracers. RESULTS: Sensorimotor cortex slices obtained from transgenic mice expressing GCaMP6 funder the Thy1 promoter were used to analyze the spontaneous calcium transients in layer 5 pyramidal neurons. Distinct subgroups of CS neurons projecting to dorsal horn and ventral areas of the same segment show more synchronous activity between them than with other subgroups. CONCLUSIONS: The results indicate that CS neurons projecting to different spinal cord zones segregated into functional ensembles depending on their hodology, suggesting that a modular organization of CS outputs controls sensorimotor behaviors in a coordinated manner.

Paced mating increases the expression of µ opioid receptors in the ventromedial hypothalamus of male rats.

Paced mating induces a positive affective state which is blocked by naloxone, an opioid antagonist. Opioids are released in the medial preoptic area (mPOA) and other brain regions during sexual behavior and µ opioid receptors (MOR) are activated in males that copulate until ejaculation. The aim of the present study was to determine if paced mating increases the expression of MOR in areas involved in the control of sexual behavior in male rats. Sexually experienced rats were assigned to one of the following groups: Paced, males that paced the sexual interaction; Non-Paced, males that did not pace the sexual interaction; Control, males were able to hear, see and smell a sexually receptive female, but no physical contact was possible. Males were sacrificed 4, 8 and 12 h after the behavioral tests. The mPOA, ventromedial hypothalamus (VMH), amygdala, olfactory bulbs (OB) and cortex were dissected and expression of MOR was determined by qPCR. In the VMH, the expression of MOR increased in the Paced group compared to the Non-Paced and Control groups at 4 h. No significant differences were found in the mPOA and the amygdala regardless of the time of sacrifice. In the OB and cortex, expression of MOR was not detectable. Interestingly, we found that the expression of MOR varied at the different times of sacrifice. In conclusion, our results show that the expression of MOR increased in the ventromedial hypothalamus after paced mating and it also varied in different brain areas depending on the time of the day.

Retrieval of Inhibitory Avoidance Memory Induces Differential Transcription of arc in Striatum, Hippocampus, and Amygdala.

Similar to the hippocampus and amygdala, the dorsal striatum is involved in memory retrieval of inhibitory avoidance, a task commonly used to study memory processes. It has been reported that memory retrieval of fear conditioning regulates gene expression of arc and zif268 in the amygdala and the hippocampus, and it is surprising that only limited effort has been made to study the molecular events caused by retrieval in the striatum. To further explore the involvement of immediate early genes in retrieval, we used real-time PCR to analyze arc and zif268 transcription in dorsal striatum, dorsal hippocampus, and amygdala at different time intervals after retrieval of step-through inhibitory avoidance memory. We found that arc expression in the striatum increased 30 min after retrieval while no changes were observed in zif268 in this region. Expression of arc and zif268 also increased in the dorsal hippocampus but the changes were attributed to context re-exposure. Control procedures indicated that in the amygdala, arc and zif268 expression was not dependent on retrieval. Our data indicate that memory retrieval of inhibitory avoidance induces arc gene expression in the dorsal striatum, caused, very likely, by the instrumental component of the task. Striatal arc expression after retrieval may induce structural and functional changes in the neurons involved in this process.

PPAR Agonists Promote the Differentiation of Porcine Bone Marrow Mesenchymal Stem Cells into the Adipogenic and Myogenic Lineages.

PURPOSE: The aim of this work was to evaluate the effect of PPAR agonists on the differentiation and metabolic features of porcine mesenchymal stem cells induced to the adipogenic or myogenic lineages. METHODS: Bone marrow MSCs from neonate pigs were isolated and identified by cell proliferation, cell surface markers or the gene expression of stem cells (CD44, CD90, CD105 or Oct4 and Nanog, respectively). Cells were differentiated into adipose or muscle cells and treated with the PPAR agonists; adipogenic and myogenic differentiation was promoted by adding these compounds. The expression of PPARγ (an adipose marker) and MyoD1 and MyHC (muscle markers), metabolic changes and expression levels of metabolic enzymes involved in glycolysis, lipogenesis, lipolysis and the pentose phosphate pathway were tested by qPCR. RESULTS: MSCs from neonate pigs exhibited high proliferation and were positive for CD44, CD90 and CD105 markers and Oct4 and Nanog expression. The treatment that promoted the highest expression of PPARγ was 50 µM of conjugated linoleic acid (CLA) c9 t11 (6.44 ± 0.69-fold, p ≤ 0.0001) in the adipose differentiation, and upregulation of HX2, ACCAα, ATGL, LPL and G6DP (p ≤ 0.0001) and downregulation of PFK and ACCAß (p ≤ 0.0001) were found. For muscle differentiation, the best treatment was 50 µM of CLA c10 t12 (59.72 ± 4.72-fold, p ≤ 0.0001), and metabolic changes were upregulation of PFK, ACCAß, G6DP, CPT1 and PPARß/δ (p ≤ 0.0001), but no effect was observed with HX2 and ACCAα (p ≥ 0.05). CONCLUSIONS: Our results suggest that differentiated cells exhibit a typical cell lineage metabolism and higher efficiencies both in anabolism and catabolism.

PCR amplification of GC-rich DNA regions using the nucleotide analog N4-methyl-2'-deoxycytidine 5'-triphosphate.

GC-rich DNA regions were PCR-amplified with Taq DNA polymerase using either the canonical set of deoxynucleoside triphosphates or mixtures in which the dCTP had been partially or completely replaced by its N4-methylated analog, N4-methyl-2'-deoxycytidine 5'-triphosphate (N4me-dCTP). In the case of a particularly GC-rich region (78.9% GC), the PCR mixtures containing N4me-dCTP produced the expected amplicon in high yield, while mixtures containing the canonical set of nucleotides produced numerous alternative amplicons. For another GC-rich DNA region (80.6% GC), the target amplicon was only generated by re-amplifying a gel-purified sample of the original amplicon with N4me-dCTP-containing PCR mixtures. In a direct PCR comparison on a highly GC-rich template, mixtures containing N4me-dCTP clearly performed better than did solutions containing the canonical set of nucleotides mixed with various organic additives (DMSO, betaine, or ethylene glycol) that have been reported to resolve or alleviate problems caused by secondary structures in the DNA. This nucleotide analog was also tested in PCR amplification of DNA regions with intermediate GC content, producing the expected amplicon in each case with a melting temperature (Tm) clearly below the Tm of the same amplicon synthesized exclusively with the canonical bases.

Steroid Receptors and Aromatase Gene Expression in Different Brain Areas of Copulating and Sexually Sluggish Male Rats.

INTRODUCTION: Sexually sluggish (SS) males have been identified in several species of mammals including rats. These animals take more than 30 minutes to ejaculate; they do not ejaculate or do so inconsistently despite being tested repeatedly with sexually receptive females. Different brain areas and hormones play an important role in the control of male sexual behavior. AIMS: Determine gene expression for the androgen receptor (AR), the estrogen receptor alpha (ERα), the progesterone receptor (PR), and the aromatase enzyme (ARO), in brain regions important in the control of male sexual behavior including the medial preoptic area (MPOA), the amygdala (AMG), the olfactory bulb (OB), and, as a control, the cortex (CTX) of copulating (C) and SS male rats. METHODS: Males that ejaculated within 30 minutes in three tests with receptive females were included in the C group, while those males that ejaculated in one or none of the four tests were included in the SS group. RNA was isolated 1 week after the last test of sexual behavior, and cDNA was synthesized from the brain areas listed above. MAIN OUTCOMES MEASURES: Expression of the AR, ERα, PR, and ARO genes was determined by quantitative polymerase chain reaction (qPCR). Cyclophilin A (CycA) and tyrosine 3-monooxygenase-tryptophan activation protein zeta (Ywhaz) were housekeeping genes used to determine relative gene expression with the 2(-ΔΔCt) method. RESULTS: The expression of mRNA for AR and ARO increased in the MPOA of SS males. ARO mRNA was increased in the AMG of SS males. In the OB, ERα mRNA was increased and AR mRNA reduced in SS males. CONCLUSION: These results indicate SS and C males show differences in gene expression within brain regions controlling sexual behavior.

Hybridisation of N4-methylcytosine-containing amplicons on DNA microarrays.

5'-Cy5-labelled PCR amplicons containing the analogue base, N(4)-methylcytosine, instead of cytosines were compared in microarray hybridisation experiments with the corresponding amplicons containing the canonical set of bases, with respect to the intensity of the fluorescence signal obtained, and cross hybridisation to non-corresponding probes. In general, higher hybridisation temperatures resulted in reduced signal intensities, particularly in the case of the N(4)-methylcytosine containing amplicons. At the lower hybridisation temperatures tested (40 °C, 30 °C), these modified amplicons gave about equal or stronger fluorescence signal than the corresponding regular amplicons. With the two GC-richest amplicons tested, in one instance the N(4)-methylated target gave a dramatically higher signal intensity than the unmodified amplicon, interpreted as reflecting the reduced formation of hairpin structures in the target sequence, due to the lower thermodynamic stability of the G:N(4)-methylC base pair, making the target more accessible, while in the other case no hybridisation was observed with either version of the amplicon, probably due to interference from a G-tetrad structure. Both for the regular and the N(4)-methylated amplicons, no significant cross hybridisation was seen in these experiments.

Capacity of N4-methyl-2'-deoxycytidine 5'-triphosphate to sustain the polymerase chain reaction using various thermostable DNA polymerases.

The dCTP analog N4-methyl-2'-deoxycytidine 5'-triphosphate (N4medCTP) was evaluated for its performance in the polymerase chain reaction (PCR). Using the HotStart Taq DNA polymerase with a standard thermal protocol, test segments 85 or 200 bp long were amplified equally well using dCTP or N4medCTP:dCTP mixtures ranging in molar ratio from 3:1 to 10:1, while complete replacement of dCTP by N4medCTP gave clearly lower amplicon yields and higher Cq values. Comparable yields with N4medCTP or dCTP were achieved only by using a slowdown protocol. Post-PCR melting analyses showed decreasing Tm values for amplicons obtained with increasing N4medCTP:dCTP input ratios; for the 200-bp amplicon, complete replacement of dCTP by N4medCTP in the reaction reduced the Tm by 11 °C; for the 85-bp amplicon the Tm reduction was 7 °C. In experiments aiming at the 200-bp amplicon, Pfu exo(-) DNA polymerase did not sustain PCR when dCTP was fully replaced by N4medCTP, even with the slowdown protocol, except at elevated N4medCTP concentrations, and, compared to PCR conducted exclusively with dCTP, the use of N4medCTP:dCTP mixtures gave reduced yields and distinctly higher Cq values, regardless of the thermal program employed. PCR experiments with 9°N DNA polymerase using N4medCTP in the conventional thermal protocol failed to produce the 200-bp amplicon.

In vitro conversion of ß-carotene to retinal in bovine rumen fluid by a recombinant ß-carotene- 15, 15'-monooxygenase.

Pasture-fed cattle yield carcasses with yellow fat; consumers often reject the resulting meat products because they assume they come from old and/or culled animals. Recombinant bacteria expressing beta-carotene 15, 15'-monooxygenase, introduced into the rumen of the animal, might help to reduce the coloration since this enzyme converts carotene to retinal, thereby eliminating the source of yellowness. The goal of this work was to evaluate the effect of a recombinant beta-carotene 15, 15'-monooxygenase (BCMO1) from Gallus gallus, expressed in Escherichia coli. The genetically modified microbe was introduced into ruminal fluid, and carotene conversion to retinal was measured. Under optimum conditions the enzyme produced 6.8 nmol of retinal per 1 mg of protein in 1 hour at 37 °C. The data on in vitro digestibility in ruminal fluid showed no differences in beta-carotene breakdown or in retinal production (p > 0.1) between E. coli with pBAD vector alone and E. coli with pBAD/BCMO1. The pBAD/BCMO1 plasmid was stable in E. coli for 750 generations. These results indicate that the protein did not break beta-carotene into retinal in ruminal fluid, perhaps due to its location in the periplasmic space in E. coli. Future research must consider strategies to release the enzyme into the rumen environment.

Time course of retinal degeneration associated with the absence of 1, 4, 5-inositol trisphosphate receptor in Drosophila melanogaster.

The absence of the inositol trisphosphate receptor is associated with a gradual retinal degeneration in Drosophila melanogaster. To characterize the time-course profile of this process, mosaic flies expressing a null allele of the itp gene in the eye were studied by electroretinograms and electronic microscopy. Membrane contour alterations, disrupted mitochondria, altered morphology and even loss of photoreceptors were increased progressively starting 5 d after hatching, were more evident during days 10-15 and promoted highly disorganized structures thereafter. Comparison between electroretinograms recorded in wild type and mutant tissues showed progressive differences in the on and off transients as well as in the magnitude of the summed receptor potentials of photoreceptor cells from day 5 of hatching, [corrected] and the functional defects became progressively more severe. Unexpectedly, these alterations were detected not only in the non-pigmented mutant ommatidia, but also in the pigmented ommatidia, including heterozygous and twin clones expressing 1, 4, 5-inositol trisphosphate receptor (IP(3)R). To explore the mechanism underlying this degenerative process, the progression of pro-oxidant and apoptotic reactions was characterized by immunohistochemical techniques. Mutant ommatidia showed intermittent episodes of increased pro-oxidant reactions (detected as adducts of 4-hydroxy-nonenal) throughout the fly's life. Similarly, several episodes of active caspase 3, an apoptotic effector, were evident with the same time pattern. Episodes of enhanced lipid peroxidation and apoptosis were also observed in the pigmented ommatidia of the mosaic eyes. The results indicate that photoreceptors lacking IP(3)R suffer episodes of increased lipid peroxidation, which eventually perturb the retinal subcellular organization and disrupt the phototransduction process and cell viability. Pigmented ommatidia also showed a similar pattern of damage, indicating that the degenerative process is non-autonomous and is so intense that it propagated to the non-mutant retinal cells in the mosaic eyes. In conclusion, ommatidia with a null mutation of IP(3)R degenerate by a process associated with intermittent lipid peroxidation and apoptotic activities.

Enhanced inhibitory avoidance learning prevents the long-term memory-impairing effects of cycloheximide, a protein synthesis inhibitor.

Interference with activity of numerous cerebral structures produces memory deficiencies; in many instances, however, when animals are over-trained such interference becomes innocuous. Systemic administration of protein synthesis inhibitors impairs long-term retention; this effect has been interpreted to mean that protein synthesis is required for memory consolidation, though little is known about the effect of protein synthesis inhibitors on memory of enhanced learning in the rat. To further analyze the protective effect of enhanced learning against amnesic treatments, groups of Wistar rats were trained in a one-trial step-through inhibitory avoidance task, using different intensities of foot-shock during training. Cycloheximide (CXM; 2.8 mg/kg), an inhibitor of protein synthesis, was injected either 30 min before training or immediately after training. Twenty-four hours after training retention latencies were recorded. Our data showed that both pre- and post-training administration of CXM produced amnesia in those groups that had been trained with relatively low foot-shock intensities, but no impairment in retention was observed when relatively high intensities of foot-shock were administered. These and similar results lead us to conclude that protein synthesis inhibitors may interfere with memory consolidation, but their effect disappears when animals are submitted to an enhanced learning experience, calling into question the idea that protein synthesis is required for memory consolidation.

MCF-7 breast carcinoma cells express ryanodine receptor type 1: functional characterization and subcellular localization.

Breast carcinoma-derived MCF-7 cells are frequently used in biomedical research. However, few reports exist regarding the characterization of signaling mechanisms in these cancerous cells involved in intracellular Ca(2+) dynamics. Consequently, the aim of these experiments was to characterize the ryanodine receptor/Ca(2+) release channel (RyR) present in MCF-7 cells. Ryanodine (100 nM), cADPR (5 microM), and caffeine (10 mM) promoted cytoplasmic Ca(2+) mobilization; in contrast, ryanodine at inhibitory concentration (100 microM) decreased the basal Ca(2+) level. Fluorescent probes demonstrated that RyR is located mainly in endomembranes. Some degree of co-localization with inositol trisphosphate receptor (IP(3)R) was observed, whereas coincidence with thapsigargin-sensitive Ca(2+)-ATPase (SERCA) was more limited. Molecular cloning resulted in the detection exclusively of RyR isoform 1. For the first time, it is shown that MCF-7 cells express functional RyR.

Molecular basis for the impaired function of the natural F112L sorcin mutant: X-ray crystal structure, calcium affinity, and interaction with annexin VII and the ryanodine receptor.

The penta-EF hand protein sorcin participates in the modulation of Ca2+-induced calcium-release in the heart through the interaction with several Ca2+ channels such as the ryanodine receptor. The modulating activity is impaired in the recently described natural F112L mutant. The F112 residue is located at the end of the D helix next to Asp113, one of the calcium ligands in the EF3 hand endowed with the highest affinity for the metal. The F112L-sorcin X-ray crystal structure at 2.5 A resolution displays marked alterations in the EF3 hand, where the hydrogen bonding network established by Phe112 is disrupted, and in the EF1 region, which is tilted in both monomers that give rise to the dimer, the stable form of the molecule. In turn, the observed tilt is indicative of an increased flexibility of the N-terminal part of the molecule. The structural alterations result in a 6-fold decrease in calcium affinity with respect to the wild-type protein and to an even larger impairment of the interaction with annexin VII and of the ability of sorcin to interact with and inhibit ryanodine receptors. These results provide a plausible structural and functional framework that helps elucidate the phenotypic alterations of mice overexpressing F112L-sorcin.

Cloning of the bovine beta-carotene-15,15'-oxygenase and expression in gonadal tissues.

Beta-carotene-15,15'-oxygenase (betaCO), found mainly in intestinal mucosa and liver, is the enzyme responsible for cleaving beta-carotene into retinal, which can be used or stored at these sites or carried by the bloodstream to different target cells within the body. We isolated the cDNA for bovine betaCO and demonstrated its expression in gonadal tissues. A cDNA of 2130 base pairs (bp) was obtained by reverse transcriptase-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE), using degenerate oligonucleotides; the deduced protein shared an identity of 75% with its homologues from other mammalian species. In order to evaluate the expression of this enzyme, we performed RT-PCR and in situ hybridizations in the ovary and testis of bovines. RT-PCR showed the expression of betaCO in testis, ovary, and cultured granulosa cells. In situ hybridization of complete ovary and testis revealed expression in granulosa cells and the corpus luteum in the ovary and in germinal and interstitial cells in the testis. These results suggest that beta-carotene could act as a local source of retinoids, which have been shown to be important during proliferation, differentiation, and maturation of both female and male germinal cells.

The foot structure from the type 1 ryanodine receptor is required for functional coupling to store-operated channels.

In the present study we have explored structural determinants of the functional interaction between skeletal muscle ryanodine receptor (RyR1) and transient receptor potential channel 1 (TRPC1) channels expressed in Chinese hamster ovary cells. We have illustrated a functional interaction between TRPC1 channels and RyR1 for the regulation of store-operated calcium entry (SOCE) initiated after releasing calcium from a caffeine-sensitive intracellular calcium pool. RNA interference experiments directed to reduce the amount of TRPC1 protein indicate that RyR1 associates to at least two different types of store-operated channels (SOCs), one dependent and one independent of TRPC1. In contrast, bradykinin-induced SOCE is completely dependent on the presence of TRPC1 protein, as we have previously illustrated. Removing the foot structure from RyR1 results in normal caffeine-induced release of calcium from internal stores but abolishes the activation of SOCE, indicating that this structure is require for functional coupling to SOCs. The footless RyR1 protein shows a different cellular localization when compared with wild type RyR1. The later protein shows a higher percentage of colocalization with FM-464, a marker of plasma membrane. The implications of the foot structure for the functional and physical coupling to TRPC and SOCs is discussed.

Regulation of cardiac excitation-contraction coupling by sorcin, a novel modulator of ryanodine receptors.

Activation of Ca2+ release channels/ryanodine receptors (RyR) by the inward Ca2+ current (I(Ca)) gives rise to Ca(2+)-induced Ca2+ release (CICR), the amplifying Ca2+ signaling mechanism that triggers contraction of the heart. CICR, in theory, is a high-gain, self-regenerating process, but an unidentified mechanism stabilizes it in vivo. Sorcin, a 21.6 kDa Ca(2+)-binding protein, binds to cardiac RyRs with high affinity and completely inhibits channel activity. Sorcin significantly inhibits both the spontaneous activity of RyRs in quiescent cells (visualized as Ca2+ sparks) and the I(Ca)-triggered activity of RyRs that gives rise to [Ca2+]i transients. Since sorcin decreases the amplitude of the [Ca2+]i transient without affecting the amplitude of I(Ca), the overall effect of sorcin is to reduce the "gain" of excitation-contraction coupling. Immunocytochemical staining shows that sorcin localizes to the dyadic space of ventricular cardiac myocytes. Ca2+ induces conformational changes and promotes translocation of sorcin between soluble and membranous compartments, but the [Ca2+] required for the latter process (ED50 = approximately 200 microM) appears to be reached only within the dyadic space. Thus, sorcin is a potent inhibitor of both spontaneous and I(Ca)-triggered RyR activity and may play a role in helping terminate the positive feedback loop of CICR.

Regulation of cardiac excitation-contraction coupling by sorcin, a novel modulator of ryanodine receptors

Activation of Ca2+ release channels/ryanodine receptors (RyR) by the inward Ca2+ current (ICa) gives rise to Ca2+-induced Ca2+ release (CICR), the amplifying Ca2+ signaling mechanism that triggers contraction of the heart. CICR, in theory, is a high-gain, self-regenerating process, but an unidentified mechanism stabilizes it in vivo. Sorcin, a 21.6 kDa Ca2+-binding protein, binds to cardiac RyRs with high affinity and completely inhibits channel activity. Sorcin significantly inhibits both the spontaneous activity of RyRs in quiescent cells (visualized as Ca2+ sparks) and the ICa-triggered activity of RyRs that gives rise to [Ca2+]i transients. Since sorcin decreases the amplitude of the [Ca2+]i transient without affecting the amplitude of ICa, the overall effect of sorcin is to reduce the "gain" of excitation-contraction coupling. Immunocytochemical staining shows that sorcin localizes to the dyadic space of ventricular cardiac myocytes. Ca2+ induces conformational changes and promotes translocation of sorcin between soluble and membranous compartments, but the [Ca2+] required for the latter process (ED50 = ~200 mM) appears to be reached only within the dyadic space. Thus, sorcin is a potent inhibitor of both spontaneous and ICa-triggered RyR activity and may play a role in helping terminate the positive feedback loop of CICR.

Sorcin inhibits calcium release and modulates excitation-contraction coupling in the heart.

Activation of Ca2+ release channels/ryanodine receptors (RyR) by the inward Ca2+ current (ICa) gives rise to Ca2+-induced Ca2+ release (CICR), the amplifying Ca2+ signaling mechanism that triggers contraction of the heart. CICR, in theory, is a high-gain, self-regenerating process, but an unidentified mechanism stabilizes it in vivo. We reported previously (Lokuta, A. J., Meyers, M. B., Sander, P. R., Fishman, G. I., and Valdivia, H. H. (1997) J. Biol. Chem. 272, 25333-25338) that sorcin, a 22-kDa Ca2+-binding protein, binds to cardiac RyRs with high affinity and completely inhibits channel activity. Here we show that sorcin significantly inhibits both the spontaneous activity of RyRs in quiescent cells (visualized as Ca2+ sparks) and the ICa-triggered activity of RyRs that gives rise to [Ca2+]i transients. Because sorcin decreased the amplitude of the [Ca2+]i transient without affecting the amplitude or kinetics of ICa, the overall effect of sorcin was to reduce the "gain" of excitation-contraction coupling. Immunocytochemical staining shows that sorcin localizes to the dyadic space of ventricular cardiac myocytes. Ca2+ induces conformational changes and promotes translocation of sorcin between soluble and membranous compartments, but the [Ca2+] required for the latter process (ED50 = approximately 200 microM) appears to be reached only within the dyadic space. Rapid injection of 5 microM sorcin onto the cytosolic face of RyRs reconstituted in lipid bilayers resulted in complete inhibition of channel activity in < or = 20 ms. Thus, sorcin is a potent inhibitor of both spontaneous and ICa-triggered RyR activity and is kinetically capable of playing a role in terminating the positive feedback loop of CICR.