Molecular and cellular evidence for the impact of a hypertrophic cardiomyopathy-associated RAF1 variant on the structure and function of contractile machinery in bioartificial cardiac tissues

Noonan syndrome (NS), the most common among RASopathies, is caused by germline variants in genes encoding components of the RAS-MAPK pathway. Distinct variants, including the recurrent Ser257Leu substitution in RAF1, are associated with severe hypertrophic cardiomyopathy (HCM). Here, we investigated the elusive mechanistic link between NS-associated RAF1^S257L and HCM using three-dimensional cardiac bodies and bioartificial cardiac tissues generated from patient-derived induced pluripotent stem cells (iPSCs) harboring the pathogenic RAF1 c.770 C > T missense change. We characterize the molecular, structural, and functional consequences of aberrant RAF1-associated signaling on the cardiac models. Ultrastructural assessment of the sarcomere revealed a shortening of the I-bands along the Z disc area in both iPSC-derived RAF1^S257L cardiomyocytes and myocardial tissue biopsies. The aforementioned changes correlated with the isoform shift of titin from a longer (N2BA) to a shorter isoform (N2B) that also affected the active force generation and contractile tensions. The genotype-phenotype correlation was confirmed using cardiomyocyte progeny of an isogenic gene-corrected RAF1^S257L-iPSC line and was mainly reversed by MEK inhibition. Collectively, our findings uncovered a direct link between a RASopathy gene variant and the abnormal sarcomere structure resulting in a cardiac dysfunction that remarkably recapitulates the human disease.

Phospholamban pentamerization increases sensitivity and dynamic range of cardiac relaxation

AIMS

A key event in the regulation of cardiac contraction and relaxation is the phosphorylation of phospholamban (PLN) that relieves the inhibition of the sarco/endoplasmic Ca2+-ATPase SERCA2a. PLN exists in an equilibrium between monomers and pentamers. While only monomers can inhibit SERCA2a by direct interaction, the functional role of pentamers is still unclear. This study investigates the functional consequences of PLN pentamerization.

METHODS AND RESULTS

We generated transgenic mouse models expressing either a PLN mutant that cannot form pentamers (TgAFA-PLN) or wildtype PLN (TgPLN) in a PLN-deficient background. TgAFA-PLN hearts demonstrated 3-fold stronger phosphorylation of monomeric PLN, accelerated Ca2+ cycling of cardiomyocytes and enhanced contraction and relaxation of sarcomeres and whole hearts in vivo. All of these effects were observed under baseline conditions and abrogated upon inhibition of protein kinase A (PKA). Mechanistically, far western kinase assays revealed that PLN pentamers are phosphorylated by PKA directly and independent of any subunit exchange for free monomers. In vitro-phosphorylation of synthetic PLN demonstrated that pentamers even provide a preferred PKA substrate and compete with monomers for the kinase, thereby reducing monomer phosphorylation and maximizing SERCA2a inhibition. However, β-adrenergic stimulation induced strong PLN monomer phosphorylation in TgPLN hearts and sharp acceleration of cardiomyocyte Ca2+ cycling and hemodynamic values that now were indistinguishable from TgAFA-PLN and PLN-KO hearts. The pathophysiological relevance of PLN pentamerization was evaluated using transverse aortic constriction (TAC) to induce left ventricular pressure overload. Compared to TgPLN, TgAFA-PLN mice demonstrated reduced survival after TAC, impaired cardiac hemodynamics, failure to respond to adrenergic stimulation, higher heart weight, and increased myocardial fibrosis.

CONCLUSIONS

The findings show that PLN pentamerization greatly impacts on SERCA2a activity as it mediates the full range of PLN effects from maximum inhibition to full release of SERCA2a. function. This regulation is important for myocardial adaptation to sustained pressure overload.

TRANSLATIONAL PERSPECTIVE

Pentamerization of PLN adds to the regulation of cardiac contractile function and facilitates myocardial transition to an energy saving mode during resting phases. Thus, PLN pentamers would protect cardiomyocytes from energetic deficits, and they improve stress adaptation of the heart as shown for sustained pressure overload in this study. Strategies that target PLN pentamerization promise therapeutic potential in the treatment of myocardial maladaptation to stress as well as cardiac pathologies associated with altered monomer-to-pentamer ratios, e.g., cardiomyopathies due to PLN mutations, certain types of heart failure, and aged hearts.

Diabetes disturbs functional adaptation of the remote myocardium after ischemia/reperfusion

Diabetes mellitus type 2 is associated with adverse clinical outcome after myocardial infarction. To better understand the underlying causes we here investigated sarcomere protein function and its calcium-dependent regulation in the non-ischemic remote myocardium (RM) of diabetic mice (db/db) after transient occlusion of the left anterior descending coronary artery. Before and 24 h after surgery db/db and non-diabetic db/+ underwent magnetic resonance imaging followed by histological and biochemical analyses of heart tissue. Intracellular calcium transients and sarcomere function were measured in isolated cardiomyocytes. Active and passive force generation was assessed in skinned fibers and papillary muscle preparations. Before ischemia and reperfusion (I/R), beat-to-beat calcium cycling was depressed in diabetic cardiomyocytes. Nevertheless, contractile function was preserved owing to increased myofilament calcium sensitivity and higher responsiveness of myocardial force production to β-adrenergic stimulation in db/db compared to db/+. In addition, protein kinase C activity was elevated in db/db hearts leading to strong phosphorylation of the titin PEVK region and increased titin-based tension of myofilaments. I/R impaired the function of whole hearts and RM sarcomeres in db/db to a larger extent than in non-diabetic db/+, and we identified several reasons. First, the amplitude and the kinetics of cardiomyocyte calcium transients were further reduced in the RM of db/db. Underlying causes involved altered expression of calcium regulatory proteins. Diabetes and I/R additively reduced phospholamban S16-phosphorylation by 80% (P < 000.1) leading to strong inhibition of the calcium ATPase SERCA2a. Second, titin stiffening was only observed in the RM of db/+, but not in the RM of db/db. Finally, db/db myofilament calcium sensitivity and force generation upon β-adrenergic stimulation were no longer enhanced over db/+ in the RM. The findings demonstrate that impaired cardiomyocyte calcium cycling of db/db hearts is compensated by increased myofilament calcium sensitivity and increased titin-based stiffness prior to I/R. In contrast, sarcomere function of the RM 24 h after I/R is poor because both these compensatory mechanisms fail and myocyte calcium handling is further depressed.

Molecular noise filtering in the β-adrenergic signaling network by phospholamban pentamers

Phospholamban (PLN) is an important regulator of cardiac calcium handling due to its ability to inhibit the calcium ATPase SERCA. β-Adrenergic stimulation reverses SERCA inhibition via PLN phosphorylation and facilitates fast calcium reuptake. PLN also forms pentamers whose physiological significance has remained elusive. Using mathematical modeling combined with biochemical and cell biological experiments, we show that pentamers regulate both the dynamics and steady-state levels of monomer phosphorylation. Substrate competition by pentamers and a feed-forward loop involving inhibitor-1 can delay monomer phosphorylation by protein kinase A (PKA), whereas cooperative pentamer dephosphorylation enables bistable PLN steady-state phosphorylation. Simulations show that phosphorylation delay and bistability act as complementary filters that reduce the effect of random fluctuations in PKA activity, thereby ensuring consistent monomer phosphorylation and SERCA activity despite noisy upstream signals. Preliminary analyses suggest that the PLN mutation R14del could impair noise filtering, offering a new perspective on how this mutation causes cardiac arrhythmias.

4-Methylumbelliferone Attenuates Macrophage Invasion and Myocardial Remodeling in Pressure-Overloaded Mouse Hearts

[Figure: see text].

Impaired Ca2+ cycling of nonischemic myocytes contributes to sarcomere dysfunction early after myocardial infarction

Changes in the nonischemic remote myocardium of the heart contribute to left ventricular dysfunction after ischemia and reperfusion (I/R). Understanding the underlying mechanisms early after I/R is crucial to improve the adaptation of the viable myocardium to increased mechanical demands. Here, we investigated the role of myocyte Ca²⁺ handling in the remote myocardium 24 h after 60 min LAD occlusion. Cardiomyocytes isolated from the basal noninfarct-related parts of wild type mouse hearts demonstrated depressed beat-to-beat Ca²⁺ handling. The amplitude of the Ca²⁺ transients as well as the kinetics of Ca²⁺ transport were reduced by up to 25%. These changes were associated with impaired sarcomere contraction. While expression levels of Ca²⁺ regulatory proteins were unchanged in remote myocardium compared to the corresponding regions of sham-operated hearts, mobility shift analyses of phosphorylated protein showed 2.9 ± 0.4-fold more unphosphorylated phospholamban (PLN) monomers, the PLN species that inhibits the Ca²⁺ ATPase SERCA2a (P ≤ 0.001). Phospho-specific antibodies revealed normal phosphorylation of PLN at T17 in remote myocardium, but markedly reduced phosphorylation at its PKA-dependent phosphorylation site, S16 (P ≤ 0.01). The underlying cause involved enhanced activity of protein phosphatases, particularly PP2A (P ≤ 0.01). In contrast, overall PKA activity was normal. The PLN interactome, as determined by co-immunoprecipitation and mass spectrometry, and the phosphorylation state of PKA targets other than PLN were also unchanged. Isoproterenol enhanced cellular Ca²⁺ cycling much stronger in remote myocytes than in healthy controls and improved sarcomere function. We conclude that the reduced phosphorylation state of PLN at S16 impairs myocyte Ca²⁺ cycling in the remote myocardium 24 h after I/R and contributes to contractile dysfunction.

Characterization of an integral protein of the brush border membrane mediating the transport of divalent metal ions

The transport of Fe(2+) and other divalent transition metal ions across the intestinal brush border membrane (BBM) was investigated using brush border membrane vesicles (BBMVs) as a model. This transport is an energy-independent, protein-mediated process. The divalent metal ion transporter of the BBM is a spanning protein, very likely a protein channel, that senses the phase transition of the BBM, as indicated by a break in the Arrhenius plot. The transporter has a broad substrate range that includes Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+). Under physiological conditions the transport of divalent metal ions is proton-coupled, leading to the acidification of the internal cavity of BBMVs. The divalent metal ion transporter can be solubilized in excess detergent (30 mM diheptanoylphosphatidylcholine or 1% Triton X-100) and reconstituted into an artificial membrane system by detergent removal. The reconstituted membrane system showed metal ion transport characteristics similar to those of the original BBMVs. The properties of the protein described here closely resemble those of the proton-coupled divalent cation transporter (DCT1, Nramp2) described by, Nature. 388:482-488). We may conclude that a protein of the Nramp family is present in the BBM, facilitating the transport of Fe(2+) and other divalent transition metal ions.

Heavy metal effects on beta-glucosidase activity influenced by pH and buffer systems

Inhibition of beta-glucosidase activity by Cu(II), Zn(II) and Ni(II) was investigated as a function of pH and buffer type. Both factors were found to exert a strong effect on the activity of the enzyme. All three of the investigated heavy metals inhibited the enzyme activity in acetate buffer. At metal concentrations of 0.6 mM, Zn and Ni reduced the enzyme activity by 25-30% under optimal pH conditions (pH 5-5.2). Under the same conditions, Cu showed an even more pronounced inhibitory effect than Zn and Ni. In presence of 0.6 mM Cu, the enzyme activity was lowered by more than 90% in comparison to metal free systems. In contrast to these results, no enzyme inhibition was observed in citrate buffer, even in the presence of 1 mM Cu. The inhibition of beta-glucosidase activity by Cu increased with increasing pH. Inhibition by Zn and Ni was less pH-dependent in the observed pH range (pH 4-5.5). Copper caused a distinct shift in the pH optimum of enzyme activity, whereas this was not the case for Zn or Ni. The effects of buffer and pH on enzyme inhibition by Cu, Zn and Ni were successfully described using a chemical speciation model, based on the assumption that enzyme activity depends on the protonation of the amino acids at the reactive site and that enzyme activity is inhibited by complexation of the reactive sites by the heavy metal cations. The results show the importance of taking chemical conditions and speciation into account when investigating the effect of heavy metal cations on biological systems.

Iron uptake by rabbit intestinal brush border membrane vesicles involves movement through the outer surface, membrane interior, inner surface and aqueous interior

Iron uptake in rabbit brush border membrane vesicles was measured in the presence of nitrilotriacetate. The complexes formed ranged from stable mononuclear species to hydrolyzed polynuclear complexes and are considered as a good model for nutritional iron compounds with respect to their chemical reactivity. Uptake includes both binding to and penetration through the membrane. A strategy was developed to localize iron in the following four compartments: outer membrane surface, membrane interior, inner membrane surface and aqueous phase within the vesicles. Both surfaces as well as the membrane interior revealed a high metal binding capacity. After an incubation for 10 min with 182 mumol/L iron and 364 mumol/L nitrilotriacetate, 35% of total vesicle iron was found to be bound to the outer membrane surface, 34% to the inner membrane surface, and 23% was not accessible to EDTA. Thus, by adsorption of polynuclear iron complexes to the outer surface, the residence time of iron may be prolonged. The remaining 8% of total iron was in the aqueous phase within the vesicles. Nitrilotriacetate enters the rabbit vesicles in a concentration-dependent manner. As a consequence, iron concentration in the aqueous phase within the vesicles will be driven to the medium equilibrium concentration.

A comparative study on iron sources for mitochondrial haem synthesis including ferritin and models of transit pool species

The rates of reaction of various exogenic iron(III) complexes with deuteroporphyrin IX in isolated mitochondria to form deuterohaem were measured. Ferritin was shown to supply iron readily for haem synthesis if the ferritin iron was reductively mobilised by the mitochondrial respiratory chain with succinate as substrate and FMN as mediator. In contrast, polynuclear complexes of iron(III) were able to form deuterohaem without added FMN. Rates of haem formation are about five times higher for the lowest polynuclear units than for ferritin. Sorbitol, gluconate, and bovine serum albumin were used as scavengers for polynuclear complexes with restricted size. Strong chelators of iron(II) compete favourably for deuterohaem formation, which supports the multistep mechanism for haem formation suggested by a priori arguments. Rates of deuterohaem formation were measured in homologous and heterologous systems of ferritins and mitochondria. Slightly differing rates of haem formation were shown to originate in different rates of iron mobilisation from the ferritins. The lack of species specificity in the interaction of ferritin with mitochondria also shows up in the linear dependence of ferritin binding on its bulk concentration as measured using 3H-labeled ferritin. Rates of haem formation are virtually the same in mitoplasts and mitochondria which indicates insignificant influences of the outer membrane. The hypothesis of low polynuclears as major components of the intracellular transit iron pool implies that both ferritin and transit iron pool species are largely equivalent sources of iron for mitochondrial haem synthesis.

Reductive mobilisation of ferritin iron

The reductive mobilisation of iron from ferritin, the principal protein of iron storage, was studied. The kinetic characteristics of iron release by dithionite, thioglycollate, and dihydroriboflavin 5'-phosphate (FMNH2) were found to differ widely. The dependence on pH is most pronounced for the dithionite reduction which proceeds 100 times faster at pH 4 than at pH 7. The experimental data can be consistently explained in terms of specific interactions of products or educts with interfacial iron(III) hydroxide of the ferritin core. Surface complexes with the product sulfite are postulated in the dithionite reaction, and with the educt in the thioglycollate reaction. Iron(II) complexes with the radical anion FMN-. are suggested to be involved in the iron release by FMNH2. The mobilisation of iron by a series of thiols of different size and coordinative properties confirmed the importance of surface complex formation. No evidence was found for predominant effects of hindered shell penetration.

Process of infection with bacteriophage phiX174. 33. Templates for the synthesis of single-stranded deoxyribonucleic acid

The origin of the templates for the synthesis of phiX174 progeny single-stranded deoxyribonucleic acid was studied by means of the mutagenic activity associated with the decay of incorporated (3)H-labeled 5-cytosine. The results indicate that the single-strand synthesis occurs in an asymmetric semiconservative manner using as template the complementary strands of the pool of replicative from molecules accumulated during the eclipse period. These complementary strands are repeatedly used as templates, and there is no detectable preferential use of complementary strand templates made early in the eclipse versus those made late.

Cytosine to thymine transitions from decay of cytosine-5-3H in bacteriophage S13

Decay of cytosine-5-(3)H incorporated into bacteriophage S13 DNA causes a molectular rearrangement of the cytosine molecutle undergoing the decay. The molecular rearrangement produces a cytosine to thyimine coding change with at efficiency approaching one. Decay of either thymidine-(methyl)-(3)H or cytosine-6-(3)H is less than 1 percent as effective in calusing either cytosine to thymine or thymine to cytosine transitions.

The mechanism of inactivation of T4 bacteriophage by tritium decay

Coliphage T4 was used as a model system to study the mechanism of biological inactivation produced by tritium decay. Experimentally, tritiated precursors were incorporated into phage DNA (thymidine-(3)H) or into phage protein ((3)H-amino acids). The ratio of killing efficiencies for decays originating in phage DNA to those originating in phage protein was 2.6. Inactivation by decays from labeled amino acids was assumed to occur exclusively from beta-particle irradiation of phage DNA. If decays originating in DNA are due solely to irradiation of DNA, then the killing efficiencies reflect the energy transfer paths in phage DNA for decays originating in phage DNA and in the protein coat. The energy transfer paths were determined for the two cases with the help of a computer and found to be very nearly equal to the experimentally determined ratio (2.6). The killing efficiencies for decays originating in phage DNA were 0.12 and for decays originating in protein 0.046.

Calculated energy deposits from the decay of tritium and other radioisotopes incorporated into bacteria

Transmutation of the radioisotope tritium occurs with the production of a low energy electron, having a range in biological material similar to the dimensions of a bacterium. A computer program was written to determine the radiation dose distributions which may be expected within a bacterium as a result of tritium decay, when the isotope has been incorporated into specific regions of the bacterium. A nonspherical model bacterium was used, represented by a cylinder with hemispherical ends. The energy distributions resulting from a wide variety of simulated labeled regions were determined; the results suggested that the nuclear region of a bacterium receives on the average significantly different per decay doses, if the labeled regions were those conceivably produced by the incorporation of thymidine-(3)H, uracil-(3)H, or (3)H-amino acids. Energy distributions in the model bacterium were also calculated for the decay of incorporated (14)carbon, (35)sulfur, and (32)phosphorous.