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1. Distinct features of calcium handling and β‐adrenergic sensitivity in heart failure with preserved versus reduced ejection fraction

   https://doi.org/10.1113/JP280425  

   Kilfoil, Peter J. ; Lotteau, Sabine ; Zhang, Rui ; Yue, Xin ; Aynaszyan, Stephan ; Solymani, Ryan E. ; Cingolani, Eugenio ; Marbán, Eduardo ; Goldhaber, Joshua I. ( September 2020 , The Journal of Physiology)

   Heart failure (HF), the leading cause of death in developed countries, occurs in the setting of reduced (HFrEF) or preserved (HFpEF) ejection fraction. Unlike HFrEF, there are no effective treatments for HFpEF, which accounts for ∼50% of heart failure.

   Abnormal intracellular calcium dynamics in cardiomyocytes have major implications for contractility and rhythm, but compared to HFrEF, very little is known about calcium cycling in HFpEF.

   We used rat models of HFpEF and HFrEF to reveal distinct differences in intracellular calcium regulation and excitation‐contraction (EC) coupling.

   While HFrEF is characterized by defective EC coupling at baseline, HFpEF exhibits enhanced coupling fidelity, further aggravated by a reduction in β‐adrenergic sensitivity.

   These differences in EC coupling and β‐adrenergic sensitivity may help explain why therapies that work in HFrEF are ineffective in HFpEF.

   Heart failure with reduced or preserved ejection fraction (respectively, HFrEF and HFpEF) is the leading cause of death in developed countries. Although numerous therapies improve outcomes in HFrEF, there are no effective treatments for HFpEF. We studied phenotypically verified rat models of HFrEF and HFpEF to compare excitation‐contraction (EC) coupling and protein expression in these two forms of heart failure. Dahl salt‐sensitive rats were fed a high‐salt diet (8% NaCl) from 7 weeks of age to induce HFpEF. Impaired diastolic relaxation and preserved ejection fraction were confirmed in each animal echocardiographically, and clinical signs of heart failure were documented. To generate HFrEF, Sprague‐Dawley (SD) rats underwent permanent left anterior descending coronary artery ligation which, 8–10 weeks later, led to systolic dysfunction (verified echocardiographically) and clinical signs of heart failure. Calcium (Ca²⁺) transients were measured in isolated cardiomyocytes under field stimulation or patch clamp. Ultra‐high‐speed laser scanning confocal imaging captured Ca²⁺sparks evoked by voltage steps. Western blotting and PCR were used to assay changes in EC coupling protein and RNA expression. Cardiomyocytes from rats with HFrEF exhibited impaired EC coupling, including decreased Ca²⁺transient (CaT) amplitude and defective couplon recruitment, associated with transverse (t)‐tubule disruption. In stark contrast, HFpEF cardiomyocytes showed saturated EC coupling (increasedI_Ca, high probability of couplon recruitment with greater Ca²⁺release synchrony, increased CaT) and preserved t‐tubule integrity. β‐Adrenergic stimulation of HFpEF myocytes with isoprenaline (isoproterenol) failed to elicit robust increases inI_Caor CaT and relaxation kinetics. Fundamental differences in EC coupling distinguish HFrEF from HFpEF.

    

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2. A chloride channel blocker prevents the suppression by inorganic phosphate of the cytosolic calcium signals that control muscle contraction

   https://doi.org/10.1113/JP279917  

   Ferreira, Juan J. ; Pequera, Germán ; Launikonis, Bradley S. ; Ríos, Eduardo ; Brum, Gustavo ( October 2020 , The Journal of Physiology)

    Accumulation of inorganic phosphate (P_i) may contribute to muscle fatigue by precipitating calcium salts inside the sarcoplasmic reticulum (SR). Neither direct demonstration of this process nor definition of the entry pathway of P_iinto SR are fully established.

    We showed that P_ipromoted Ca²⁺release at concentrations below 10 mmand decreased it at higher concentrations. This decrease correlated well with that of [Ca²⁺]_SR.

    Pre‐treatment of permeabilized myofibres with 2 mmCl⁻channel blocker 9‐anthracenecarboxylic acid (9AC) inhibited both effects of P_i.

    The biphasic dependence of Ca²⁺release on [P_i] is explained by a direct effect of P_iacting on the SR Ca²⁺release channel, combined with the intra‐SR precipitation of Ca²⁺salts. The effects of 9AC demonstrate that P_ienters the SR via a Cl⁻pathway of an as‐yet‐undefined molecular nature.

   Fatiguing exercise causes hydrolysis of phosphocreatine, increasing the intracellular concentration of inorganic phosphate (P_i). P_idiffuses into the sarcoplasmic reticulum (SR) where it is believed to form insoluble Ca²⁺salts, thus contributing to the impairment of Ca²⁺release. Information on the P_ientrance pathway is still lacking. In amphibian muscles endowed with isoform 3 of the RyR channel, Ca²⁺spark frequency is correlated with the Ca²⁺load of the SR and can be used to monitor this variable. We studied the effects of P_ion Ca²⁺sparks in permeabilized fibres of the frog. Relative event frequency (f/f_ref) rose with increasing [P_i], reaching 2.54 ± 1.6 at 5 mm,and then decreased monotonically, reaching 0.09 ± 0.03 at [P_i] = 80 mm. Measurement of [Ca²⁺]_SRconfirmed a decrease correlated with spark frequency at high [P_i]. A large [Ca²⁺]_SRsurge was observed upon P_iremoval. Anion channels are a putative path for P_iinto the SR. We tested the effect of the chloride channel blocker 9‐anthracenecarboxylic acid (9AC) on P_ientrance. 9AC (400 µm)applied to the cytoplasm produced a non‐significant increase in spark frequency and reduced the P_ieffects on this parameter. Fibre treatment with 2 mm9AC in the presence of high cytoplasmic Mg²⁺suppressed the effects of P_ion [Ca²⁺]_SRand spark frequency up to 55 mm[P_i]. These results suggest that chloride channels (or transporters) provide the main pathway of inorganic phosphate into the SR and confirm that P_iimpairs Ca²⁺release by accumulating and precipitating with Ca²⁺inside the SR, thus contributing to myogenic fatigue.

    

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3. Designer genetically encoded voltage‐dependent calcium channel inhibitors inspired by RGK GTPases

   https://doi.org/10.1113/JP276544  

   Colecraft, Henry M. ( April 2020 , The Journal of Physiology)

   High‐voltage‐activated calcium (Ca_V1/Ca_V2) channels translate action potentials into Ca²⁺influx in excitable cells to control essential biological processes that include; muscle contraction, synaptic transmission, hormone secretion and activity‐dependent regulation of gene expression. Modulation of Ca_V1/Ca_V2 channel activity is a powerful mechanism to regulate physiology, and there are a host of intracellular signalling molecules that tune different aspects of Ca_Vchannel trafficking and gating for this purpose. Beyond normal physiological regulation, the diverse Ca_Vchannel modulatory mechanisms may potentially be co‐opted or interfered with for therapeutic benefits. Ca_V1/Ca_V2 channels are potently inhibited by a four‐member sub‐family of Ras‐like GTPases known as RGK (Rad, Rem, Rem2, Gem/Kir) proteins. Understanding the mechanisms by which RGK proteins inhibit Ca_V1/Ca_V2 channels has led to the development of novel genetically encoded Ca_Vchannel blockers with unique properties; including, chemo‐ and optogenetic control of channel activity, and blocking channels either on the basis of their subcellular localization or by targeting an auxiliary subunit. These genetically encoded Ca_Vchannel inhibitors have outstanding utility as enabling research tools and potential therapeutics.image

    

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4. Plant “helper” immune receptors are Ca 2+ -permeable nonselective cation channels

   https://doi.org/10.1126/science.abg7917  

   Jacob, Pierre ; Kim, Nak Hyun ; Wu, Feihua ; El-Kasmi, Farid ; Chi, Yuan ; Walton, William G. ; Furzer, Oliver J. ; Lietzan, Adam D. ; Sunil, Sruthi ; Kempthorn, Korina ; et al ( June 2021 , Science)

   Plant nucleotide-binding leucine-rich repeat receptors (NLRs) regulate immunity and cell death. InArabidopsis, a subfamily of “helper” NLRs is required by many “sensor” NLRs. Active NRG1.1 oligomerized, was enriched in plasma membrane puncta, and conferred cytoplasmic calcium ion (Ca²⁺) influx in plant and human cells. NRG1.1-dependent Ca²⁺influx and cell death were sensitive to Ca²⁺channel blockers and were suppressed by mutations affecting oligomerization or plasma membrane enrichment. Ca²⁺influx and cell death mediated by NRG1.1 and ACTIVATED DISEASE RESISTANCE 1 (ADR1), another helper NLR, required conserved negatively charged N-terminal residues. Whole-cell voltage-clamp recordings demonstrated thatArabidopsishelper NLRs form Ca²⁺-permeable cation channels to directly regulate cytoplasmic Ca²⁺levels and consequent cell death. Thus, helper NLRs transduce cell death signals directly.

    

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5. Synergid cell calcium oscillations refine understanding of FERONIA/LORELEI signaling during interspecific hybridization

   https://doi.org/10.1007/s00497-023-00483-6  

   Ponvert, Nathaniel ; Johnson, Mark A. ( November 2023 , Plant Reproduction)

   Pollen tubes from closely related species and mutants lacking pollen tube MYB transcription factors are able to initiate FER/LRE-dependent synergid cell calcium oscillations.

   Reproductive isolation leads to the evolution of new species; however, the molecular mechanisms that maintain reproductive barriers between sympatric species are not well defined. In flowering plants, sperm cells are immotile and are delivered to female gametes by the pollen grain. After landing on the stigmatic surface, the pollen grain germinates a polarized extension, the pollen tube, into floral tissue. After growing via polar extension to the female gametes and shuttling its cargo of sperm cells through its cytoplasm, the pollen tube signals its arrival and identity to synergid cells that flank the egg. If signaling is successful, the pollen tube and receptive synergid cell burst, and sperm cells are released for fusion with female gametes. To better understand cell–cell recognition during reproduction and how reproductive barriers are maintained between closely related species, pollen tube-initiated synergid cell calcium ion dynamics were examined during interspecific crosses. It was observed that interspecific pollen tubes successfully trigger synergid cell calcium oscillations—a hallmark of reproductive success—but signaling fails downstream of key signaling genes and sperm are not released. This work further defines pollen tube–synergid cell signaling as a critical block to interspecific hybridization and suggests that the FERONIA/LORELEI signaling mechanism plays multiple parallel roles during pollen tube reception.

    

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