SR Ca2+refilling upon depletion and SR Ca2+uptake rates during development in rabbit ventricular myocytes

Molecular dynamics: a powerful tool for studying the medicinal chemistry of ion channel modulators

Molecular dynamics (MD) simulations allow researchers to investigate the behavior of desired biological targets at ever-decreasing costs with ever-increasing precision. Among the biological macromolecules, ion channels are remarkable transmembrane proteins, capable of performing special biological processes and revealing a complex regulatory matrix, including modulation by small molecules, either endogenous or exogenous. Recently, given the developments in ion channel structure determination and accessibility of bio-computational techniques, MD and related tools are becoming increasingly popular in the intense research area regarding ligand-channel interactions. This review synthesizes and presents the most important fields of MD involvement in investigating channel-molecule interactions, including, but not limited to, deciphering the binding modes of ligands to their ion channel targets and the mechanisms through which chemical compounds exert their effect on channel function. Special attention is devoted to the importance of more elaborate methods, such as free energy calculations, while principles regarding drug design and discovery are highlighted. Several technical aspects involving the creation and simulation of channel-molecule MD systems (ligand parameterization, proper membrane setup, system building, etc.) are also presented.

Structure and assembly mechanisms of toxic human islet amyloid polypeptide oligomers associated with copper

The molecular interaction of hIAPP with Cu(ii) mediates the formation of off-pathway and toxic oligomers which have small-sized and random coil structures.

Amyloidosis is a clinical disorder implicated with the formation of toxic amyloid aggregates. Despite their pathological significance, it is challenging to define the structural characteristics of amyloid oligomers owing to their metastable nature. Herein, we report structural and mechanistic investigations of human islet amyloid polypeptide (hIAPP) oligomers, found in type II diabetes mellitus, in both the absence and presence of disease-relevant metal ions [i.e., Cu(ii) and Zn(ii)]. These metal ions show suppressive effects on hIAPP fibrillation and facilitate the generation of toxic oligomers. Using circular dichroism spectroscopy, transmission electron microscopy, gel electrophoresis, small-angle X-ray scattering, and ion mobility-mass spectrometry, we investigated the assembly mechanisms of hIAPP oligomers in the presence and absence of metal ions. Oligomerization of both metal-free hIAPP and metal-associated hIAPP monomers is initiated following a similar growth model. However, in the presence of Cu(ii), hIAPP monomers self-assemble into small globular aggregates (R_g ∼ 45 Å) with a random coil structure. This Cu(ii)-associated hIAPP oligomer shows an off-pathway aggregation, and is suggested to be an end product which is toxic to pancreatic β-cells. On the other hand, metal-free hIAPP and Zn(ii)-associated hIAPP monomers generate relatively less toxic aggregates that eventually grow into fibrils. We suggest that the coordination of hIAPP to Cu(ii) and the relatively high stability (K_a, ca. 10⁸ M^–1) of hIAPP–Cu(ii) complexes result in the abnormal conformation and toxicity of hIAPP oligomers. Overall, through combining multiple biophysical methods, our studies suggest that molecular interactions between hIAPP and Cu(ii) induce a different pathway for hIAPP assembly. This work will advance our knowledge of the conformational basis, assembly mechanism, and toxicity of small soluble amyloid oligomers.

Function and regulation of the Na+-Ca2+ exchanger NCX3 splice variants in brain and skeletal muscle

Isoform 3 of the Na(+)-Ca(2+) exchanger (NCX3) is crucial for maintaining intracellular calcium ([Ca(2+)]i) homeostasis in excitable tissues. In this sense NCX3 plays a key role in neuronal excitotoxicity and Ca(2+) extrusion during skeletal muscle relaxation. Alternative splicing generates two variants (NCX3-AC and NCX3-B). Here, we demonstrated that NCX3 variants display a tissue-specific distribution in mice, with NCX3-B as mostly expressed in brain and NCX-AC as predominant in skeletal muscle. Using Fura-2-based Ca(2+) imaging, we measured the capacity and regulation of the two variants during Ca(2+) extrusion and uptake in different conditions. Functional studies revealed that, although both variants are activated by intracellular sodium ([Na(+)]i), NCX3-AC has a higher [Na(+)]i sensitivity, as Ca(2+) influx is observed in the presence of extracellular Na(+). This effect could be partially mimicked for NCX3-B by mutating several glutamate residues in its cytoplasmic loop. In addition, NCX3-AC displayed a higher capacity of both Ca(2+) extrusion and uptake compared with NCX3-B, together with an increased sensitivity to intracellular Ca(2+). Strikingly, substitution of Glu(580) in NCX3-B with its NCX3-AC equivalent Lys(580) recapitulated the functional properties of NCX3-AC regarding Ca(2+) sensitivity, Lys(580) presumably acting through a structure stabilization of the Ca(2+) binding site. The higher Ca(2+) uptake capacity of NCX3-AC compared with NCX3-B is in line with the necessity to restore Ca(2+) levels in the sarcoplasmic reticulum during prolonged exercise. The latter result, consistent with the high expression in the slow-twitch muscle, suggests that this variant may contribute to the Ca(2+) handling beyond that of extruding Ca(2+).

STAT1-mediated Bim expression promotes the apoptosis of retinal pericytes under high glucose conditions

Hyperglycemia impacts different vascular cell functions and promotes the development and progression of various vasculopathies including diabetic retinopathy. Although the increased rate of apoptosis in pericytes (PCs) has been linked to increased oxidative stress and activation of protein kinase C-δ (PKC-δ) and SHP-1 (Src homology region 2 domain-containing phosphatase-1) tyrosine phosphatase during diabetes, the detailed mechanisms require further elucidation. Here we show that the rate of apoptosis and expression of proapoptotic protein Bim were increased in the retinal PCs of diabetic Akita/+ mice and mouse retinal PCs cultured under high glucose conditions. Increased Bim expression in retinal PCs under high glucose conditions required the sustained activation of signal transducer and activator of transcription 1 (STAT1) through production of inflammatory cytokines. PCs cultured under high glucose conditions also exhibited increased oxidative stress and diminished migration. Inhibition of oxidative stress, PKC-δ or Rho-associated protein kinase I/II was sufficient to protect PCs against apoptosis under high glucose conditions. Furthermore, PCs deficient in Bim expression were protected from high glucose-mediated increased oxidative stress and apoptosis. However, only inhibition of PKC-δ lowered Bim levels. N-acetylcysteine did not affect STAT1 levels, suggesting that oxidative stress is downstream of Bim. PCs cultured under high glucose conditions disrupted capillary morphogenesis of retinal endothelial cells (ECs) in coculture experiments. In addition, conditioned medium prepared from PCs under high glucose conditions attenuated EC migration. Taken together, our results indicate that Bim has a pivotal role in the dysfunction of retinal PCs under high glucose conditions by increasing oxidative stress and death of PCs.

Detection, properties, and frequency of local calcium release from the sarcoplasmic reticulum in teleost cardiomyocytes

Calcium release from the sarcoplasmic reticulum (SR) plays a central role in the regulation of cardiac contraction and rhythm in mammals and humans but its role is controversial in teleosts. Since the zebrafish is an emerging model for studies of cardiovascular function and regeneration we here sought to determine if basic features of SR calcium release are phylogenetically conserved. Confocal calcium imaging was used to detect spontaneous calcium release (calcium sparks and waves) from the SR. Calcium sparks were detected in 16 of 38 trout atrial myocytes and 6 of 15 ventricular cells. The spark amplitude was 1.45±0.03 times the baseline fluorescence and the time to half maximal decay of sparks was 27±3 ms. Spark frequency was 0.88 sparks µm(-1) min(-1) while calcium waves were 8.5 times less frequent. Inhibition of SR calcium uptake reduced the calcium transient (F/F(0)) from 1.77±0.17 to 1.12±0.18 (p = 0.002) and abolished calcium sparks and waves. Moreover, elevation of extracellular calcium from 2 to 10 mM promoted early and delayed afterdepolarizations (from 0.6±0.3 min(-1) to 8.1±2.0 min(-1), p = 0.001), demonstrating the ability of SR calcium release to induce afterdepolarizations in the trout heart. Calcium sparks of similar width and duration were also observed in zebrafish ventricular myocytes. In conclusion, this is the first study to consistently report calcium sparks in teleosts and demonstrate that the basic features of calcium release through the ryanodine receptor are conserved, suggesting that teleost cardiac myocytes is a relevant model to study the functional impact of abnormal SR function.

Calcium handling in zebrafish ventricular myocytes

The zebrafish is an important model for the study of vertebrate cardiac development with a rich array of genetic mutations and biological reagents for functional interrogation. The similarity of the zebrafish (Danio rerio) cardiac action potential with that of humans further enhances the relevance of this model. In spite of this, little is known about excitation-contraction coupling in the zebrafish heart. To address this issue, adult zebrafish cardiomyocytes were isolated by enzymatic perfusion of the cannulated ventricle and were subjected to amphotericin-perforated patch-clamp technique, confocal calcium imaging, and/or measurements of cell shortening. Simultaneous recordings of the voltage dependence of the L-type calcium current (I(Ca,L)) amplitude and cell shortening showed a typical bell-shaped current-voltage (I-V) relationship for I(Ca,L) with a maximum at +10 mV, whereas calcium transients and cell shortening showed a monophasic increase with membrane depolarization that reached a plateau at membrane potentials above +20 mV. Values of I(Ca,L) were 53, 100, and 17% of maximum at -20, +10, and +40 mV, while the corresponding calcium transient amplitudes were 64, 92, and 98% and cell shortening values were 62, 95, and 96% of maximum, respectively, suggesting that I(Ca,L) is the major contributor to the activation of contraction at voltages below +10 mV, whereas the contribution of reverse-mode Na/Ca exchange becomes increasingly more important at membrane potentials above +10 mV. Comparison of the recovery of I(Ca,L) from acute and steady-state inactivation showed that reduction of I(Ca,L) upon elevation of the stimulation frequency is primarily due to calcium-dependent I(Ca,L) inactivation. In conclusion, we demonstrate that a large yield of healthy atrial and ventricular myocytes can be obtained by enzymatic perfusion of the cannulated zebrafish heart. Moreover, zebrafish ventricular myocytes differed from that of large mammals by having larger I(Ca,L) density and a monophasically increasing contraction-voltage relationship, suggesting that caution should be taken upon extrapolation of the functional impact of mutations on calcium handling and contraction in zebrafish cardiomyocytes.

Association of highly compact type II diabetes related islet amyloid polypeptide intermediate species at physiological temperature revealed by diffusion NMR spectroscopy

Self-association of human islet amyloid polypeptide (hIAPP) is correlated with the development of type II diabetes by the disruption of cellular homeostasis in islet cells through the formation of membrane-active oligomers. The toxic species of hIAPP responsible for membrane damage has not been identified. In this study, we show by pulsed field gradient NMR spectroscopy that the monomeric form of the toxic, amyloidogenic human variant of IAPP (hIAPP) adopts a temperature dependent compact folded conformation that is absent in both the nontoxic and nonamyloidogenic rat variant of IAPP and absent in hIAPP at low temperatures, suggesting this compact form of monomeric hIAPP may be linked to its later aggregation and cytotoxicity. In addition to the monomeric form of hIAPP, a large oligomeric species greater than 100 nm in diameter is also present but does not trigger the nucleation-dependent aggregation of IAPP at 4 degrees C, indicating the large oligomeric species may be an off-pathway intermediate that has been predicted by kinetic models of IAPP fiber formation. Furthermore, analysis of the polydispersity of the calculated diffusion values indicates small oligomeric species of hIAPP are absent in agreement with a recent ultracentrifugation study. The absence of small oligomeric species in solution suggests the formation of small, well-defined ion channels by hIAPP may proceed by aggregation of monomeric IAPP on the membrane, rather than by the insertion of preformed structured oligomers from the solution state as has been proposed for other amyloidogenic proteins.

Three-dimensional structure and orientation of rat islet amyloid polypeptide protein in a membrane environment by solution NMR spectroscopy

Islet amyloid polypeptide (IAPP or amylin) is a 37-residue peptide hormone associated with glucose metabolism that is cosecreted with insulin by beta-cells in the pancreas. Since human IAPP is a highly amyloidogenic peptide, it has been suggested that the formation of IAPP amyloid fibers is responsible for the death of beta-cells during the early stages of type II diabetes. It has been hypothesized that transient membrane-bound alpha-helical structures of human IAPP are precursors to the formation of these amyloid deposits. On the other hand, rat IAPP forms transient alpha-helical structures but does not progress further to form amyloid fibrils. To understand the nature of this intermediate state and the difference in toxicity between the rat and human versions of IAPP, we have solved the high-resolution structure of rat IAPP in the membrane-mimicking detergent micelles composed of dodecylphosphocholine. The structure is characterized by a helical region spanning the residues A5 to S23 and a disordered C-terminus. A distortion in the helix is seen at R18 and S19 that may be involved in receptor binding. Paramagnetic quenching NMR experiments indicate that rat IAPP is bound on the surface of the micelle, in agreement with other nontoxic forms of IAPP. A comparison to the detergent-bound structures of other IAPP variants indicates that the N-terminal region may play a crucial role in the self-association and toxicity of IAPP by controlling access to the putative dimerization interface on the hydrophobic face of the amphipathic helix.

Ca2+ sparks and cellular distribution of ryanodine receptors in developing cardiomyocytes from rat

Although abundant ryanodine receptors (RyRs) exist in cardiomyocytes from newborn (NB) rat and despite the maturity of their single-channel properties, the RyR contribution to excitation-contraction (E-C) coupling is minimal. Immature arrangement of RyRs in the Ca(2+) release site of the sarcoplasmic reticulum and/or distant RyRs location from the sarcolemmal Ca(2+) signal could explain this quiescence. Consequently, Ca(2+) sparks and their cellular distribution were studied in NB myocytes and correlated with the formation of dyads and transverse (T) tubules. Ca(2+) sparks were recorded in fluo-4-loaded intact ventricular myocytes acutely dissociated from adult and NB rats (0-9 days old). Sparks were defined/compared in the center and periphery of the cell. Co-immunolocalization of RyRs with dihydropyridine receptors (DHPR) was used to estimate dyad formation, while the development of T tubules was studied using di-8-ANEPPS and diIC12. Our results indicate that in NB cells, Ca(2+) sparks exhibited lower amplitude (1.7+/-0.5 vs. 3.6+/-1.7 F/F(0)), shorter duration (47+/-3.2 vs. 54.1+/-3 ms), and larger width (1.7+/-0.8 vs. 1.2+/-0.4 microm) than in adult. Although no significant changes were observed in the overall frequency, central sparks increased from approximately 60% at 0-1 day to 82% at 7-9 days. While immunolocalization revealed many central release sites at 7-8 days, fluorescence labeling of the plasma membrane showed less abundant internal T tubules. This could imply that although during the first week, release sites emerge forming dyads with DHPR-containing T tubules; some of these T tubules may not be connected to the surface, explaining the RyR quiescence during E-C coupling in NB.