Characterisation of thapsigargin-releasable Ca(2+) from the Ca(2+)-ATPase of sarcoplasmic reticulum at limiting [Ca(2+)]

Gene Panel Sequencing Identifies a Novel RYR1 p.Ser2300Pro Variant as Candidate for Malignant Hyperthermia with Multi-Minicore Myopathy

Malignant hyperthermia (MH), a rare autosomal dominant pharmacogenetic disorder of skeletal muscle calcium regulation, is triggered by sevoflurane in susceptible individuals. We report a Korean having MH with multi-minicore myopathy functionally supported by RYR1-mediated intracellular Ca2+ release testing in B lymphocytes. A 14-year-old boy was admitted for the evaluation of progressive torticollis accompanied by cervicothoracic scoliosis. During the preoperative drape of the patient for the release of the sternocleidomastoid muscle under general anesthesia, his wrist and ankle were observed to have severe flexion contracture. The body temperature was 37.1 °C. To treat MH, the patient was administered a bolus of dantrolene intravenously (1.5 mg/kg) and sodium bicarbonate. After a few minutes, muscle rigidity, tachycardia, and EtCO2 all resolved. Next-generation panel sequencing for hereditary myopathy identified a novel RYR1 heterozygous missense variant (NM_000540.2: c.6898T > C; p.Ser2300Pro), which mapped to the MH2 domain of the protein, a hot spot for MH mutations. Ex vivo RYR1-mediated intracellular Ca2+ release testing in B lymphocytes showed hypersensitive Ca2+ responses to isoflurane and caffeine, resulting in an abnormal Ca2+ release only in the proband, not in his family members. Our findings expand the clinical and pathological spectra of information associated with MH with multi-minicore myopathy.

The role of potassium and host calcium signaling in Toxoplasma gondii egress

Toxoplasma gondii is an obligate intracellular parasite and replicates inside a parasitophorous vacuole (PV) within the host cell. The membrane of the PV (PVM) contains pores that permits for equilibration of ions and small molecules between the host cytosol and the PV lumen. Ca²⁺ signaling is universal and both T. gondii and its mammalian host cell utilize Ca²⁺ signals to stimulate diverse cellular functions. Egress of T. gondii from host cells is an essential step for the infection cycle of T. gondii, and a cytosolic Ca²⁺ increase initiates a Ca²⁺ signaling cascade that culminates in the stimulation of motility and egress. In this work we demonstrate that intracellular T. gondii tachyzoites are able to take up Ca²⁺ from the host cytoplasm during host cell signaling events. Both intracellular and extracellular Ca²⁺ sources are important in reaching a threshold of parasite cytosolic Ca²⁺ needed for successful egress. Two peaks of Ca²⁺ were observed in egressing single parasites with the second peak resulting from Ca²⁺ entry. We patched infected host cells to allow the delivery of precise concentrations of Ca²⁺ for the stimulation of motility and egress. Using this approach of patching infected host cells, allowed us to determine that increasing the host cytosolic Ca²⁺ to a specific concentration can trigger egress, which is further accelerated by diminishing the concentration of potassium (K⁺).

The anti-apoptotic protein HAX-1 interacts with SERCA2 and regulates its protein levels to promote cell survival

Cardiac contractility is regulated through the activity of various key Ca(2+)-handling proteins. The sarco(endo)plasmic reticulum (SR) Ca(2+) transport ATPase (SERCA2a) and its inhibitor phospholamban (PLN) control the uptake of Ca(2+) by SR membranes during relaxation. Recently, the antiapoptotic HS-1-associated protein X-1 (HAX-1) was identified as a binding partner of PLN, and this interaction was postulated to regulate cell apoptosis. In the current study, we determined that HAX-1 can also bind to SERCA2. Deletion mapping analysis demonstrated that amino acid residues 575-594 of SERCA2's nucleotide binding domain are required for its interaction with the C-terminal domain of HAX-1, containing amino acids 203-245. In transiently cotransfected human embryonic kidney 293 cells, recombinant SERCA2 was specifically targeted to the ER, whereas HAX-1 selectively concentrated at mitochondria. On triple transfections with PLN, however, HAX-1 massively translocated to the ER membranes, where it codistributed with PLN and SERCA2. Overexpression of SERCA2 abrogated the protective effects of HAX-1 on cell survival, after hypoxia/reoxygenation or thapsigargin treatment. Importantly, HAX-1 overexpression was associated with down-regulation of SERCA2 expression levels, resulting in significant reduction of apparent ER Ca(2+) levels. These findings suggest that HAX-1 may promote cell survival through modulation of SERCA2 protein levels and thus ER Ca(2+) stores.

Transplantation of Myocyte Precursors Derived from Embryonic Stem Cells Transfected with IGFII Gene in a Mouse Model of Muscle Injury

BACKGROUND

Reconstruction of skeletal muscle tissue is hampered by the lack of availability of functional substitution of the tissue.

METHODS

Embryonic stem (ES) cells were transfected with the insulin-like growth factor (IGF) II gene and were selected with G418. The resultant cell clones were analyzed regarding their myogenic differentiation in vitro and in vivo.

RESULTS

The cells expressed early and late myogenic differentiation markers, including myoD, myogenin, and dystrophin in vitro. They had phosphorylated Akt within the cells, suggesting their activation by the secreted IGFII. Transplantation of the cells to injured anterior tibial muscle of mice significantly improved their motor functions compared to injured mice transplanted with undifferentiated ES cells and injured mice given vehicle alone. The transfected cells adapted to the injured muscle, formed myofibers positive for dystrophin and negative for MyoD and myogenin. Trichrome staining and toluidine blue staining support myofiber formation in vivo. The enzymatic activity of acetylcholine esterase suggested the functional activity of the regenerated motor units. The evoked electromyogram of anterior tibial muscle transplanted with the transfected cells showed significantly higher potentials compared to that transplanted with undifferentiated ES cells and that injected with phosphate-buffered saline (control injury). Electron microscopic examination confirmed the myofiber formation in the cells in vivo.

CONCLUSIONS

Transfection of IGFII gene into ES cells may be applicable for transplantation therapy of muscle damage due to injury and myopathies.

Flubendiamide, a novel Ca2+ channel modulator, reveals evidence for functional cooperation between Ca2+ pumps and Ca2+ release

Flubendiamide, developed by Nihon Nohyaku Co., Ltd. (Tokyo, Japan), is a novel activator of ryanodine-sensitive calcium release channels (ryanodine receptors; RyRs), and is known to stabilize insect RyRs in an open state in a species-specific manner and to desensitize the calcium dependence of channel activity. In this study, using flubendiamide as an experimental tool, we examined an impact of functional modulation of RyR on Ca2+ pump. Strikingly, flubendiamide induced a 4-fold stimulation of the Ca2+ pump activity (EC50=11 nM) of an insect that resequesters Ca2+ to intracellular stores, a greater increase than with the classical RyR modulators ryanodine and caffeine. This prominent stimulation, which implies tight functional coupling of Ca2+ release with Ca2+ pump, resulted in a marginal net increase in the extravesicular calcium concentration despite robust Ca2+ release from the intracellular stores by flubendiamide. Further analysis suggested that luminal Ca2+ is an important mediator for the functional coordination of RyRs and Ca2+ pumps. However, kinetic factors for Ca2+ pumps, including ATP and cytoplasmic Ca2+, failed to affect the Ca2+ pump stimulation by flubendiamide. We therefore conclude that the stimulation of Ca2+ pump by flubendiamide is mediated by the decrease in luminal calcium, which may induce calcium dissociation from the luminal Ca2+ binding site on the Ca2+ pump. This mechanism should play an essential role in precise control of intracellular Ca2+ homeostasis.

Lyso-glycosphingolipids mobilize calcium from brain microsomes via multiple mechanisms

Recently, we demonstrated that the GSL (glycosphingolipid), GlcCer (glucosylceramide), modulates Ca2+ release from intracellular stores and from microsomes by sensitizing the RyaR (ryanodine receptor), a major Ca2+-release channel of the endoplasmic reticulum, whereas the lyso derivative of GlcCer, namely GlcSph (glucosylsphingosine), induced Ca2+ release via a mechanism independent of the RyaR [Lloyd-Evans, Pelled, Riebeling, Bodennec, de-Morgan, Waller, Schiffmann and Futerman (2003) J. Biol. Chem. 278, 23594-23599]. We now systematically examine the mechanism by which GlcSph and other lyso-GSLs modulate Ca2+ mobilization from rat brain cortical and cerebellar microsomes. GlcSph, lactosylsphingosine and galactosylsphingosine all mobilized Ca2+, but at significantly higher concentrations than those required for GlcCer-mediated sensitization of the RyaR. GlcSph-induced Ca2+ mobilization was partially blocked by heparin, an inhibitor of the Ins(1,4,5) P3 receptor, and also partially blocked by thapsigargin or ADP, inhibitors of SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase), but completely blocked when both acted together. In contrast, neither lactosylsphingosine nor galactosylsphingosine had any effect on Ca2+ release via either the Ins(1,4,5) P3 receptor or SERCA, but acted as agonists of the RyaR. Finally, and surprisingly, all three lyso-GSLs reversed inhibition of SERCA by thapsigargin. We conclude that different lyso-GSLs modulate Ca2+ mobilization via different mechanisms, and discuss the relevance of these findings to the GSL storage diseases in which lyso-GSLs accumulate.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.