Danicamtiv Increases Myosin Recruitment and Alters Cross-Bridge Cycling in Cardiac Muscle

BACKGROUND

Modulating myosin function is a novel therapeutic approach in patients with cardiomyopathy. Danicamtiv is a novel myosin activator with promising preclinical data that is currently in clinical trials. While it is known that danicamtiv increases force and cardiomyocyte contractility without affecting calcium levels, detailed mechanistic studies regarding its mode of action are lacking.

METHODS

Permeabilized porcine cardiac tissue and myofibrils were used for X-ray diffraction and mechanical measurements. A mouse model of genetic dilated cardiomyopathy was used to evaluate the ability of danicamtiv to correct the contractile deficit.

RESULTS

Danicamtiv increased force and calcium sensitivity via increasing the number of myosins in the ON state and slowing cross-bridge turnover. Our detailed analysis showed that inhibition of ADP release results in decreased cross-bridge turnover with cross bridges staying attached longer and prolonging myofibril relaxation. Danicamtiv corrected decreased calcium sensitivity in demembranated tissue, abnormal twitch magnitude and kinetics in intact cardiac tissue, and reduced ejection fraction in the whole organ.

CONCLUSIONS

As demonstrated by the detailed studies of Danicamtiv, increasing myosin recruitment and altering cross-bridge cycling are 2 mechanisms to increase force and calcium sensitivity in cardiac muscle. Myosin activators such as Danicamtiv can treat the causative hypocontractile phenotype in genetic dilated cardiomyopathy.

Danicamtiv increases myosin recruitment and alters the chemomechanical cross bridge cycle in cardiac muscle

Modulating myosin function is a novel therapeutic approach in patients with cardiomyopathy. Detailed mechanism of action of these agents can help predict potential unwanted affects and identify patient populations that can benefit most from them. Danicamtiv is a novel myosin activator with promising preclinical data that is currently in clinical trials. While it is known danicamtiv increases force and cardiomyocyte contractility without affecting calcium levels, detailed mechanistic studies regarding its mode of action are lacking. Using porcine cardiac tissue and myofibrils we demonstrate that Danicamtiv increases force and calcium sensitivity via increasing the number of myosin in the "on" state and slowing cross bridge turnover. Our detailed analysis shows that inhibition of ADP release results in decreased cross bridge turnover with cross bridges staying on longer and prolonging myofibril relaxation. Using a mouse model of genetic dilated cardiomyopathy, we demonstrated that Danicamtiv corrected calcium sensitivity in demembranated and abnormal twitch magnitude and kinetics in intact cardiac tissue.

Significance Statement

Directly augmenting sarcomere function has potential to overcome limitations of currently used inotropic agents to improve cardiac contractility. Myosin modulation is a novel mechanism for increased contraction in cardiomyopathies. Danicamtiv is a myosin activator that is currently under investigation for use in cardiomyopathy patients. Our study is the first detailed mechanism of how Danicamtiv increases force and alters kinetics of cardiac activation and relaxation. This new understanding of the mechanism of action of Danicamtiv can be used to help identify patients that could benefit most from this treatment.

0655 CBT-I and CPAP in Comorbid Insomnia and Sleep Apnea: Effects on Daytime Functioning

Introduction

This study examines the effects of treatment sequences using cognitive-behavioral therapy for insomnia (CBT-I) and continuous positive airway pressure (CPAP) therapy on daytime functioning in people with comorbid insomnia and sleep apnea (COMISA).

Methods

118 participants with COMISA (Age=49.99±13.12; 53.4% female) were randomized to one of the three study arms: Arm A- CBT-I followed by CPAP, Arm B- CBT-I concurrent with CPAP, and Arm C- CPAP only. Participants were assessed at four time points [baseline/ start of phase 1 (A1), CPAP titration/ start of phase 2 (A2), 30 days (A3) and 90 days (A4) after CPAP initiation]. This study examined secondary outcome measures of daytime functioning, including the Functional Outcomes of Sleep Questionnaire (FOSQ), Epworth Sleepiness Scale, and Flinders Fatigue Scale (FFS).

Results

Linear mixed model analyses showed a main effect of time on improving functional outcomes in all measurements, with all p< 0.001. There were also arm by time interactions on FOSQ [F(6, 105.36)=4.21, p=0.001] and FFS scores [F(6, 106.95)=3.10, p=0.008]. Pairwise comparisons with Bonferroni adjustment showed improved FOSQ scores in Arm A from A1 to A2 (p=0.011) and A2 to A3 (p=0.005), Arm B from A2 to A3 (p< 0.001), and Arm C from A2 to A3 (p=0.006). For FFS scores, improvements were shown in Arm A from A1 to A2 (p=0.003), and Arm B from A2 to A3 (p < 0.001).

Conclusion

The results show daytime functioning improvements in patients with COMISA following CPAP and CBT-I. In addition, CBT-I appears to facilitate improvement in sleepiness-related functional status and daytime fatigue. The findings suggest that the combination of CBT-I and CPAP may have a beneficial effect on daytime functioning in patients with COMISA.

Support

This study was supported by the National Institutes of Health (R01HL114529).

Functional analysis of the transcriptional activity of the mouse phospholipid transfer protein gene

Phospholipid transfer protein (PLTP) plays an important role in the metabolism of plasma high density lipoprotein. The mouse gene encoding PLTP and its promoter region has been cloned in our laboratory. The present study was conducted to functionally analyze the transcriptional regulation of the mouse PLTP gene. The results indicated that DNA sequences between -245 and -69 were responsible for the full promoter activity and binding motifs for transcription factor Sp1 and AP-2 within this functional promoter region were synergistically essential for the basal transcription. The transcriptional activity of this gene was significantly increased by chenodeoxycholic acid and fenofibrate, suggesting that transcription factor farnesoid X-activated receptor (FXR) and peroxisome proliferator-activated receptor (PPAR) are likely involved in the transcriptional regulation. DNA sequence analysis suggests that DNA sequences from -407 to -395 and from -393 to -381 are homologous to the recognition motifs of FXR, and those from -859 to -847 and from -309 to -297 are similar to the potential binding motif for PPAR. These findings provide a molecular basis for further investigation of the physiological function and regulation of the PLTP gene in mice.

Glucose regulates the transcription of human genes relevant to HDL metabolism: responsive elements for peroxisome proliferator-activated receptor are involved in the regulation of phospholipid transfer protein

Phospholipid transfer protein (PLTP) plays an important role in human plasma HDL metabolism. Clinical data have recently indicated that plasma PLTP activity and mass were both higher in diabetic patients concomitant with hyperglycemia. The present study shows that high glucose increases both PLTP mRNA and functional activity in HepG2 cells, due to a significant increase in the promoter activity of human PLTP gene. The glucose-responsive elements are located between -759 and -230 of the PLTP 5'-flanking region, within which two binding motifs (-537 to -524 and -339 to -327) for either peroxisome proliferator-activated receptor or farnesoid X-activated receptor are involved in this glucose-mediated transcriptional regulation. This finding suggests that high glucose upregulates the transcription of human PLTP gene via nuclear hormone receptors. In addition, high glucose increases mRNA levels for several genes that are functionally important in HDL metabolism, including human ATP-binding cassette transporter A1, apolipoprotein A-I, scavenger receptor BI, and hepatic lipase. The functional promoter activities of these genes are enhanced by high glucose in three cell lines tested, indicating that glucose may also regulate these genes at the transcriptional level. Our findings provide a molecular basis for a role of hyperglycemia in altered HDL metabolism.

DNA sequences responsible for reduced promoter activity of human phospholipid transfer protein by fibrate

Phospholipid transfer protein (PLTP) plays an important role in plasma lipid and lipoprotein metabolism. We have previously cloned and characterized the promoter region of the human PLTP gene. The present study was conducted to determine if the promoter activity of the human PLTP gene is affected by fibrate, a hypolipidemic drug, and to identify DNA sequences that are responsible for the effect. The results indicated that the promoter activity of the PLTP gene was significantly reduced by fenofibrate, and the area that was mainly responsive to the reducing effect by fibrate was located between -377 and -230 of the 5'-flanking region. The DNA sequence analysis suggested that each area of the DNA sequences from -342 to -323 and from -322 to -299 has two repeated sequences, which are inverted and homologous to the recognition motif of peroxisome proliferator-activated receptor (PPAR), namely the PPAR-responsive element (PPRE). Mutagenesis of these PPRE-like sequences, especially that at -322 to -299, abolished most of the reducing effects of fibrate on the PLTP promoter activity. These findings strongly suggest that the PPRE-like elements are responsible for the reduced promoter activity of the human PLTP gene by fibrate.

5-HT(2A/2C) receptor antagonists in the paraventricular nucleus attenuate the action of DOI on NPY-stimulated eating

Hypothalamic neuropeptide Y (NPY) and serotonin (5-HT)-containing neurons are believed to exert an interactive effect on ingestive behavior. The present study examined the ability of two serotonergic antagonists, spiperone (SPIP), a 5-HT2A antagonist, and mianserin (MIAN), a 5-HT(2A/2C) antagonist, to block the inhibitory action of the 5-HT(2A/2C) receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on NPY-stimulated eating. Drugs were injected directly into the paraventricular nucleus (PVN), the perifornical (PFH) or the ventromedial hypothalamus (VMH) at the onset of the dark cycle. PVN, PFH and VMH injections of NPY potentiated food intake although only PVN pretreatment with DOI (5-20 nmol) suppressed NPY-induced eating. SPIP or MIAN, injected immediately prior to PVN DOI, reversed the suppressive effect of DOI on NPY feeding. These findings are consistent with other recent data showing that 5-HT2A receptors within the PVN modulate NPY's effect on food intake at the start of the nocturnal period.

Introduction of the human PLTP transgene suppresses the atherogenic diet-induced increase in plasma phospholipid transfer activity in C57BL/6 mice

The human plasma phospholipid transfer protein (PLTP) has been shown to facilitate the transfer of phospholipids between lipoproteins and convert high-density lipoproteins into larger and smaller particles in vitro. To explore the lipid transport function in vivo, transgenic C57BL/6 mice that express the human PLTP gene, driven by its natural promoter, were generated. Little difference in PLTP activity and lipoprotein lipids was observed between transgenic mice and non-transgenic control mice fed the chow diet. In response to an atherogenic high-fat, high-cholesterol, cholic acid containing diet, the PLTP activity increased significantly with time in control mice (62% in males and 34% in females after the high-fat diet for 18 weeks). In contrast, the PLTP activity did not change appreciably in the transgenic mice fed the atherogenic diet. Thus, the introduction of the human transgene suppressed the diet-induced increase in plasma PLTP activity, as evidenced by a decrease in PLTP mRNA in a variety of tissues. High-density lipoprotein levels decreased in mice fed the atherogenic diet, but there was a proportionally greater decrease in transgenic animals than in controls. After 18 weeks on the atherogenic diet, the transgenic animals had high-density lipoprotein-cholesterol and PLTP activity approximately one-half of that of control animals. Non-denaturing gradient gel electrophoresis of plasma indicated that the atherogenic diet decreased the high-density lipoprotein size distribution in control mice. However, high-density lipoprotein particle size distribution of the transgenic mice was shifted to smaller particles compared with control animals (P < 0.001). These findings suggest that PLTP activity can modulate the effects of an atherogenic diet on high-density lipoproteins.

Relationship between phospholipid transfer protein activity and HDL level and size among inbred mouse strains

Because of the paucity of data on phospholipid transfer protein (PLTP) activity and lipoprotein phospholipid in mouse strains, plasma PLTP activity (PLTA), plasma phospholipid and cholesterol, HDL phospholipid and cholesterol, and HDL size distribution were determined in 15 inbred mouse strains. The 15 inbred mouse strains differed in their relatedness to one another and consisted of six largely unrelated groups: Castaneus, Swiss, C57BL, AKR, DBA, and NZB. Lipid and PLTA analyses were performed on plasma pools from male and female mice that had fasted for 4 h prior to blood draw. Among the representative unrelated strains fed the chow diet, there was a highly significant relationship between PLTA and plasma phospholipid (r(s) = 0.727, P < 0.01), HDL phospholipid (r(s) = 0.762, P < 0.01), HDL cholesterol (r(s) = 0.699, P < 0.02), percentage of large HDL particles (r(s) = 0.699, P < 0.02), and HDL peak size (r(s) = 0.776, P < 0.01). Similar results were obtained among these strains fed a high fat, high cholesterol diet. PLTA increased in all strains fed the high fat diet (chix = 94%, range 6 to 221%). Strain SM having relatively low PLTA and HDL was crossed with strain NZB having high PLTA and HDL. The F1 progeny from this cross were backcrossed to strain SM and 41 male backcross progeny collected. Among these individual backcrossed animals, PLTA was highly correlated with plasma phospholipid (r(s) = 0.508, P = 0.001), HDL phospholipid (r(s) = 0.566, P < 0.001), HDL cholesterol (r(s) = 0.532, P < 0.001), and percentage of large HDL particles (r(s) = 0.446, P = 0.020). Therefore, we conclude that PLTP is a determinant of HDL level and size in mice.-Albers, J. J., W. Pitman, G. Wolfbauer, M. C. Cheung, H. Kennedy, A-Y. Tu, S. M. Marcovina, and B. Paigen. Relationship between phospholipid transfer protein activity and HDL level and size among inbred mouse strains.

Characterization of the mouse gene encoding phospholipid transfer protein

The mouse gene encoding phospholipid transfer protein (PLTP) was cloned from 129/SvJ lambdaFIX(R) II library and characterized for the first time. It is comprised of 16 exons separated by 15 introns. Its gene organization strikingly resembles that encoding the human PLTP; the exon-intron junctions in these two genes are completely conserved. Sequencing analysis reveals that the putative promoter of mouse PLTP gene consists of a TATA-box, a high GC region, and several consensus sequences for the binding of transcription factors. Within the first 200 bp of the 5'-flanking region, the mouse and human PLTP genes share 81.1% identity of nt sequences and contain the consensus sequences for the transcription factors AP-2 and Sp1 at the same locations.

DNA sequences essential for transcription of human phospholipid transfer protein gene in HepG2 cells

The present study was conducted to determine the essential DNA sequences required for the transcription of the human phospholipid transfer protein gene. Truncation studies revealed that DNA sequences between -230 and -159, particularly those at the upstream region, were responsible for the full promoter activity. This region was able to compete with AP-2 and GRE oligonucleotides for the binding to HepG2 cell nuclear extract as shown by gel mobility shift assay. Further analysis, using site-directed mutagenesis, indicated that DNA sequences identical to Sp1 and highly homologous to GRE and Ap-2 consensus sequences were essential for the transcription. These findings support the concept that several elements, spread over the entire functional promoter, synergistically drive the basal transcription.

Transgenic mice expressing human phospholipid transfer protein have increased HDL/non-HDL cholesterol ratio

The role of plasma phospholipid transfer protein (PLTP) in lipoprotein metabolism is poorly understood. In vitro studies suggest that PLTP influences HDL size and composition and transfers phospholipids among lipoproteins. To provide an in vivo model for studies of PLTP physiology, transgenic mice that express human PLTP were generated. Human PLTP transcripts were detected in total RNA from adipose tissue, lung, heart, and spleen of the two distinct lines (A and C) of transgenic mice. Despite minimal expression of human PLTP in the liver of these transgenic mice and similar plasma phospholipid transfer activity in transgenic and non-transgenic mice (19.1 +/- 3.1 vs 18.9 +/- 2.7 mumol/ml/h), differences in lipoprotein levels were observed between transgenic and control mice receiving the same chow diet. Male transgenic mice of line C had significantly higher HDL cholesterol than control mice (76.4 +/- 4.6 vs 71.9 +/- 7.0 mg/dl, p < 0.05) and the male transgenic mice of lines A and C had a significantly lower non-HDL cholesterol (15.1 +/- 4.1 and 15.6 +/- 4.7 vs 20.9 +/- 5.5 mg/dl, P < 0.01 and P < 0.02) and a significantly higher HDL cholesterol/non-HDL cholesterol ratio than the control mice (5.3 +/- 1.3 and 5.5 +/- 2.2 vs 3.9 +/- 1.9 mg/dl, P < 0.01 and P < 0.02). Female mice from transgenic line C had higher HDL cholesterol than control mice (64.6 +/- 4.8 vs 57.4 +/- 5.1 mg/dl, P < 0.01) while female mice from line A tended to have higher HDL cholesterol/non-HDL cholesterol ratio than control mice (5.5 +/- 3.7 vs 3.8 +/- 1.4). These observations suggest that expression of PLTP in peripheral tissues play an important role in lipoprotein metabolism. Expression of human PLTP produced a more favorable lipoprotein profile and thus, enhanced expression of PLTP could potentially retard atherosclerosis.

Neutralization and transfer of lipopolysaccharide by phospholipid transfer protein

Phospholipid transfer protein (PLTP) and lipopolysaccharide-binding protein (LPB) are lipid transfer proteins found in human plasma. PLTP shares 24% sequence similarity with LBP. PLTP mediates the transfer and exchange of phospholipids between lipoprotein particles, whereas LBP transfers bacterial lipopolysaccharide (LPS) either to lipoprotein particles or to CD14, a soluble and cell-surface receptor for LPS. We asked whether PLTP could interact with LPS and mediate the transfer of LPS to lipoproteins or to CD14. PLTP was able to bind and neutralize LPS: incubation of LPS with purified recombinant PLTP (rPLTP) resulted in the inhibition of the ability of LPS to stimulate adhesive responses of neutrophils, and addition of rPLTP to blood inhibited cytokine production in response to LPS. Transfer of LPS by rPLTP was examined using fluorescence dequenching experiments and native gel electrophoresis. The results suggested that rPLTP was able to mediate the exchange of LPS between micelles and the transfer of LPS to reconstituted HDL particles, but it did not transfer LPS to CD14. Consonant with these findings, rPLTP did not mediate CD14-dependent adhesive responses of neutrophils to LPS. These results suggest that while PLTP and LBP both bind and transfer LPS, PLTP is unable to transfer LPS to CD14 and thus does not mediate responses of cells to LPS.

Molecular biology of phospholipid transfer protein

Lipid transfer proteins play an essential role in the intravascular dynamics of lipids among lipoproteins and between lipoproteins and cell membranes. Phospholipid transfer protein has been known for over a decade but, unlike cholesteryl ester transfer protein, has been investigated relatively little with regard to its physiological importance. The recent determination of the phospholipid transfer protein complementary DNA sequence as well as the further characterization of its gene structure will direct future studies toward the understanding of its structure-function correlations, physiological regulation, and clinical assessment at the molecular level. As a member of the lipid-transfer lipopolysaccharide-binding protein gene family, phospholipid transfer protein will attract investigators to studying its possible involvement in lipopolysaccharide or endotoxin interactions in addition to its phospholipid transfer activity.

Functional characterization of the promoter region of the human phospholipid transfer protein gene

We have identified the functional promoter region of the human phospholipid transfer protein gene. Primer extension analysis indicates multiple sites for transcription initiation. Sequence analysis reveals that the promoter consists of TATA box, high GC region, and several consensus sequences for the potential binding of transcription factors. To assay promoter activity, DNA fragments with various lengths of the 5'-flanking region were fused upstream to the luciferase gene and transfected into HepG2, COS-7, and CHO cells. A minimal promoter of 159 base pairs between -230 and -72 relative to the first transcriptional initiation site is responsible for the full activity. Two motifs, Sp1 and AP-2, are located within this area. It may suggest that the PLTP promoter activity relies primarily on the putative cis-elements in the functional region.

Functional expression of human and mouse plasma phospholipid transfer protein: effect of recombinant and plasma PLTP on HDL subspecies

The molecular cloning of mouse plasma phospholipid transfer protein (PLTP) and the eukaryotic cell expression of complementary DNA for mouse and human PLTP are described. Mouse PLTP was found to share 83% amino acid sequence identity with human PLTP. PLTP was produced in baby hamster kidney cells. Conditioned medium from BHK cells expressing PLTP possessed both phospholipid transfer activity and high density lipoprotein (HDL) conversion activity. PLTP mRNA was detected in all 16 human tissues examined by Northern blot analysis with ovary, thymus, and placenta having the highest levels. PLTP mRNA was also examined in eight mouse tissues with the highest PLTP mRNA levels found in the lung, brain, and heart. The effect of purified human plasma-derived PLTP and human recombinant PLTP (rPLTP) on the two human plasma HDL subspecies Lp(A-I) and Lp(A-I/A-II) was evaluated. Plasma PLTP or rPLTP converted the two distinct size subspecies of Lp(A-I) into a larger species, an intermediate species, and a smaller species. Lp(A-I/A-II) particles containing multiple size subspecies were significantly altered by incubation with either plasma or rPLTP with the largest but less prominent subspecies becoming the predominant one, and the smallest subspecies increasing in concentration. Thus, PLTP promoted the conversion of both Lp(A-I) and Lp(A-I/A-II) to populations of larger and smaller particles. Also, both human PLTP and mouse rPLTP were able to convert human or mouse HDL into larger and smaller particles. These observations suggest that PLTP may play a key role in extracellular phospholipid transport and modulation of HDL particles.

Organization of human phospholipid transfer protein gene

We have determined the exon/intron organization of the human phospholipid transfer protein gene. The gene, which spans approximately 13.3 kilobases, is comprised of 16 exons. The organization of the phospholipid transfer protein gene strikingly resembles that encoding another plasma lipid transfer protein, the human cholesterol ester transfer protein. The exon-intron junctions in these two genes are highly conserved, with eight out of fifteen junctions interrupting the same codons, while the remaining junctions lie within 5 residues of each other. The similarity in gene structure and homology in coding sequences suggests that these two genes most likely evolved from a common ancestral gene.

High density lipoprotein conversion mediated by human plasma phospholipid transfer protein

Phospholipid transfer protein (PLTP) was purified from lipoprotein-free human plasma, obtained upon treatment of plasma with dextran sulfate and Ca2+, by employing a series of column chromatography. Upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the purified PLTP showed a single main band, corresponding to the molecular mass of 78 kDa. However, isoelectric focusing of the purified preparation gave multiple bands with pI ranging from 4.3 to 5.1, indicative of microheterogeneity. Purified PLTP was shown to possess not only phospholipid transfer activity, but also high density lipoprotein (HDL) conversion activity (Tu, A.-Y., Nishida, H. I., and Nishida, T. (1990), FASEB J. 4, A2148; Jauhiainen, M., Metso, J., Pahlman, R., Blomqvist, S., van Tol, A., and Ehnholm, C. (1993) J. Biol. Chem. 268, 4032-4036). Isolated HDL3 was enlarged to the size of HDL2b upon incubation with purified PLTP for 6 h at 37 degrees C at the PLTP/HDL3 molar ratio of approximately 1:45. Both the HDL conversion and the phosphatidylcholine transfer activities of purified PLTP were effectively inhibited by rabbit anti-PLTP immunoglobulin G. The primary importance of PLTP in the HDL enlargement that occurs in human plasma upon incubation at 37 degrees C was shown by the strong inhibitory effect of the anti-PLTP immunoglobulin G. The process of PLTP-mediated HDL enlargement was accompanied by the release of apoproteins, primarily apoA-I. HDL3 enlargement mediated by PLTP was effectively inhibited by the addition of free fatty acids.