Multivalent dendrimeric and monomeric adenosine agonists attenuate cell death in HL-1 mouse cardiomyocytes expressing the A(3) receptor

Functionalization of soft materials for cardiac repair and regeneration

Coronary artery disease is a leading cause of death in developed nations. As the disease progresses, myocardial infarction can occur leaving areas of dead tissue in the heart. To compensate, the body initiates its own repair/regenerative response in an attempt to restore function to the heart. These efforts serve as inspiration to researchers who attempt to capitalize on the natural regenerative processes to further augment repair. Thus far, researchers are exploiting these repair mechanisms in the functionalization of soft materials using a variety of growth factor-, ligand- and peptide-incorporating approaches. The goal of functionalizing soft materials is to best promote and direct the regenerative responses that are needed to restore the heart. This review summarizes the opportunities for the use of functionalized soft materials for cardiac repair and regeneration, and some of the different strategies being developed.

The potential for nanotechnology to improve delivery of therapy to the acute ischemic heart

Treatment of acute cardiac ischemia remains an area in which there are opportunities for therapeutic improvement. Despite significant advances, many patients still progress to cardiac hypertrophy and heart failure. Timely reperfusion is critical in rescuing vulnerable ischemic tissue and is directly related to patient outcome, but reperfusion of the ischemic myocardium also contributes to damage. Overproduction of reactive oxygen species, initiation of an inflammatory response and deregulation of calcium homeostasis all contribute to injury, and difficulties in delivering a sufficient quantity of drug to the affected tissue in a controlled manner is a limitation of current therapies. Nanotechnology may offer significant improvements in this respect. Here, we review recent examples of how nanoparticles can be used to improve delivery to the ischemic myocardium, and suggest some approaches that may lead to improved therapies for acute cardiac ischemia.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Programmable nanoscaffolds that control ligand display to a G-protein-coupled receptor in membranes to allow dissection of multivalent effects

A programmable ligand display system can be used to dissect the multivalent effects of ligand binding to a membrane receptor. An antagonist of the A_2A adenosine receptor, a G-protein-coupled receptor that is a drug target for neurodegenerative conditions, was displayed in 35 different multivalent configurations, and binding to A_2A was determined. A theoretical model based on statistical mechanics was developed to interpret the binding data, suggesting the importance of receptor dimers. Using this model, extended multivalent arrangements of ligands were constructed with progressive improvements in binding to A_2A. The results highlight the ability to use a highly controllable multivalent approach to determine optimal ligand valency and spacing that can be subsequently optimized for binding to a membrane receptor. Models explaining the multivalent binding data are also presented.

Conditioned medium from hypoxic cells protects cardiomyocytes against ischemia

The hypothesis of the present study is that cardiomyocytes subjected to prolonged ischemia, may release survival factors that will protect new cardiac cells from ischemic stress. We exposed neonatal rat cardiomyocyte primary cultures to hypoxia, collected the supernatant, treated intact cardiac cells by this posthypoxic supernatant, and exposed them to hypoxia. The results show cardioprotection of the treated cells compared with the untreated ones. We named the collected posthypoxic supernatant "conditioned medium" (CM), which acts in a dose-dependent manner to protect new cardiac cells from hypoxia: 100 or 75% of CM diluted in phosphate-buffered saline (PBS) protected cells as if they were not exposed to hypoxia (P < 0.001). When CM was removed from the cells before hypoxia, protection was not observed. CM also protected skeletal muscle cultures from hypoxia, but not cardiac cells against H(2)O(2)-induced cell damage. Finally, CM treatment protected the isolated heart in Langendorff set-up against ischemia. Smaller infarct size (9.9 ± 4.4% vs. 28.3 ± 8.5%, P < 0.05), better Rate Pressure Product (67 ± 11% vs. 48.6 ± 13.4%, P < 0.05) and better rate of contraction and relaxation were observed following ischemia and reperfusion (1341 ± 399 mmHg/s vs. 951 ± 349 mmHg/s, P < 0.05 and 1053 ± 347 mmHg/s vs. 736 ± 314 mmHg/s, P < 0.05). To conclude, there are factors that are released from the heart cells subjected to ischemia/hypoxia that protects cardiomyocytes from ischemic stress.

Anti-ischemic effects of multivalent dendrimeric A₃ adenosine receptor agonists in cultured cardiomyocytes and in the isolated rat heart

Adenosine released during myocardial ischemia mediates cardioprotective preconditioning. Multivalent drugs covalently bound to nanocarriers may differ greatly in chemical and biological properties from the corresponding monomeric agents. Here, we conjugated chemically functionalized nucleosides to poly(amidoamine) (PAMAM) dendrimeric polymers and investigated their effects in rat primary cardiac cell cultures and in the isolated heart. Three conjugates of A₃ adenosine receptor (AR) agonists, chain-functionalized at the C2 or N⁶ position, were cardioprotective, with greater potency than monomeric agonist Cl-IB-MECA. Multivalent amide-linked MRS5216 was selective for A₁ and A₃ARs, and triazole-linked MRS5246 and MRS5539 (optionally containing fluorescent label) were A₃AR-selective. The conjugates protected ischemic rat cardiomyocytes, an effect blocked by an A₃AR antagonist MRS1523, and isolated hearts with significantly improved infarct size, rate of pressure product, and rate of contraction and relaxation. Thus, strategically derivatized nucleosides tethered to biocompatible polymeric carriers display enhanced cardioprotective potency via activation of A₃AR on the cardiomyocyte surface.

GPCR ligand dendrimer (GLiDe) conjugates: adenosine receptor interactions of a series of multivalent xanthine antagonists

Previously, G protein-coupled receptor (GPCR) agonists were tethered from polyamidoamine (PAMAM) dendrimers to provide high receptor affinity and selectivity. Here, we prepared GPCR ligand--dendrimer (GLiDe) conjugates from a potent adenosine receptor (AR) antagonist; such agents are of interest for treating Parkinson's disease, asthma, and other conditions. Xanthine amine congener (XAC) was appended with an alkyne group on an extended C8 substituent for coupling by Cu(I)-catalyzed click chemistry to azide-derivatized G4 (fourth-generation) PAMAM dendrimers to form triazoles. These conjugates also contained triazole-linked PEG groups (8 or 22 moieties per 64 terminal positions) for increasing water-solubility and optionally prosthetic groups for spectroscopic characterization and affinity labeling. Human AR binding affinity increased progressively with the degree of xanthine substitution to reach K(i) values in the nanomolar range. The order of affinity of each conjugate was hA(2A)AR > hA(3)AR > hA(1)AR, while the corresponding monomer was ranked hA(2A)AR > hA(1)AR ≥ hA(3)AR. The antagonist activity of the most potent conjugate 14 (34 xanthines per dendrimer) was examined at the G(i)-coupled A(1)AR. Conjugate 14 at 100 nM right-shifted the AR agonist concentration--response curve in a cyclic AMP functional assay in a parallel manner, but at 10 nM (lower than its K(i) value), it significantly suppressed the maximal agonist effect in calcium mobilization. This is the first systematic probing of a potent AR antagonist tethered on a dendrimer and its activity as a function of variable loading.

GPCR ligand-dendrimer (GLiDe) conjugates: future smart drugs?

Unlike nanocarriers that are intended to release their drug cargo at the site of action, biocompatibile polyamidoamine (PAMAM) conjugates are designed to act at cell surface G protein-coupled receptors (GPCRs) without drug release. These multivalent GPCR ligand-dendrimer (GLiDe) conjugates display qualitatively different pharmacological properties in comparison with monomeric drugs. They might be useful as novel tools to study GPCR homodimers and heterodimers as well as higher aggregates. The structure of the conjugate determines the profile of biological activity, receptor selectivity, and physical properties such as water solubility. Prosthetic groups for characterization and imaging of receptors can be introduced without loss of affinity. The feasibility of targeting multiple adenosine and P2Y receptors for synergistic effects has been shown. Testing in vivo will be needed to explore the effects on pharmacokinetics and tissue targeting.