Conformationally Controlled Oligocholate Membrane Transporters: Learning through Water Play

Controlled translocation of molecules and ions across lipid membranes is the basis of numerous biological functions. Because synthetic systems can help researchers understand the more complex biological ones, many chemists have developed synthetic mimics of biological transporters. Both systems need to deal with similar fundamental challenges. In addition to providing mechanistic insights into transport mechanisms, synthetic transporters are useful in a number of applications including separation, sensing, drug delivery, and catalysis

Diabetes Alters Intracellular Calcium Transients in Cardiac Endothelial Cells

Diabetic cardiomyopathy (DCM) is a diabetic complication, which results in myocardial dysfunction independent of other etiological factors. Abnormal intracellular calcium ([Ca2+]i) homeostasis has been implicated in DCM and may precede clinical manifestation. Studies in cardiomyocytes have shown that diabetes results in impaired [Ca2+]i homeostasis due to altered sarcoplasmic reticulum Ca2+ ATPase (SERCA) and sodium-calcium exchanger (NCX) activity. Importantly, altered calcium homeostasis may also be involved in diabetes-associated endothelial dysfunction, including impaired endothelium-dependent relaxation and a diminished capacity to generate nitric oxide (NO), elevated cell adhesion molecules, and decreased angiogenic growth factors. However, the effect of diabetes on Ca2+ regulatory mechanisms in cardiac endothelial cells (CECs) remains unknown. The objective of this study was to determine the effect of diabetes on [Ca2+]i homeostasis in CECs in the rat model (streptozotocin-induced) of DCM. DCM-associated cardiac fibrosis was confirmed using picrosirius red staining of the myocardium. CECs isolated from the myocardium of diabetic and wild-type rats were loaded with Fura-2, and UTP-evoked [Ca2+]i transients were compared under various combinations of SERCA, sarcoplasmic reticulum Ca2+ ATPase (PMCA) and NCX inhibitors. Diabetes resulted in significant alterations in SERCA and NCX activities in CECs during [Ca2+]i sequestration and efflux, respectively, while no difference in PMCA activity between diabetic and wild-type cells was observed. These results improve our understanding of how diabetes affects calcium regulation in CECs, and may contribute to the development of new therapies for DCM treatment

Translocation of Hydrophilic Molecules across Lipid Bilayers by Salt-Bridged Oligocholates

Macrocyclic oligocholates were found in a previous work (Cho, H.; Widanapathirana, L.; Zhao, Y. J. Am. Chem. Soc.2011, 133, 141−147) to stack on top of one another in lipid membranes to form nanopores. Pore formation was driven by a strong tendency of the water molecules in the interior of the amphiphilic macrocycles to aggregate in a nonpolar environment. In this work, cholate oligomers terminated with guanidinium and carboxylate groups were found to cause efflux of hydrophilic molecules such as glucose, maltotriose, and carboxyfluorescein (CF) from POPC/POPG liposomes. The cholate trimer outperformed other oligomers in the transport. Lipid-mixing assays and dynamic light scattering ruled out fusion as the cause of leakage. The strong dependence on chain length argues against random intermolecular aggregates as the active transporters. The efflux of glucose triggered by these compounds increased significantly when the bilayers contained 30 mol % cholesterol. Hill analysis suggested that the active transporter consisted of four molecules. The oligocholates were proposed to fold into “noncovalent macrocycles” by the guanidinium−carboxylate salt bridge and stack on top of one another to form similar transmembrane pores as their covalent counterparts.Reprinted (adapted) with permission from Langmuir 27 (2011): 4936, doi:10.1021/la2005166. Copyright 2011 American Chemical Society.</p

Environmental Effects Dominate the Folding of Oligocholates in Solution, Surfactant Micelles, and Lipid Membranes

Oligocholate foldamers with different numbers and locations of guanidinium−carboxylate salt bridges were synthesized. The salt bridges were introduced by incorporating arginine and glutamic acid residues into the foldamer sequence. The conformations of these foldamers were studied by fluorescence spectroscopy in homogeneous solution, anionic and nonionic micelles, and lipid bilayers. Environmental effects instead of inherent foldability were found to dominate the folding. As different noncovalent forces become involved in the conformations of the molecules, the best folder in one environment could turn into the worst in another. Preferential solvation was the main driving force for the folding of oligocholates in solution. The molecules behaved very differently in micelles and lipid bilayers, with the most critical factors controlling the folding−unfolding equilibrium being the solvation of ionic groups and the abilities of the surfactants/lipids to compete for the salt bridge. Because of their ability to fold into helices with a nonpolar exterior and a polar interior, the oligocholates could transport large hydrophilic molecules such as carboxyfluorescein across lipid bilayers. Both the conformational properties of the oligocholates and their binding with the guest were important to the transport efficiency.Reprinted (adapted) with permission from Journal of the American Chemical Society 132 (2010): 9890, doi:10.1021/ja103694p. Copyright 2010 American Chemical Society.</p