Biophysical studies of the interaction of squalamine and other cationic amphiphilic molecules with bacterial and eukaryotic membranes: importance of the distribution coefficient in membrane selectivity
Introduction
Emergence of pathogenic bacteria exhibiting resistance against multiple conventional antibiotics has led to the development of new antimicrobial molecules. Among such molecules, cationic amphiphilic (amphipathic) antimicrobials (CAAs) seem to be attractive candidates (Savage et al., 2002).
CAAs are structurally unrelated molecules that are positively charged and amphipathic. CAAs include cationic surfactants, cyclic lipopeptides, β-sheet and α-helix-forming peptides and cholesterol derivatives (Savage et al., 2002). The antibacterial activity of CAAs relies on their ability to interact with bacterial lipids, especially lipopolysaccharide (LPS) and phosphatidylglycerol (PG), resulting in: (i) the release of divalent cations bridging LPS molecules together, (ii) a change in the outer membrane permeability barrier, (iii) the release of membrane vesicle, (iv) membrane lipid exchange, which in turn causes membrane permeabilisation and bacteria killing (Katsu et al., 1984, Katsu et al., 2002, Savage et al., 2002, Clausell et al., 2004, Clausell et al., 2005, Clausell et al., 2007, McBroom and Kuehn, 2007). Binding of CAAs to bacterial membranes involves both electrostatic and hydrophobic interactions (Savage et al., 2002). The conserved cationic charge of CAAs allows them to bind to the anionic charge of lipopolysaccharide (LPS) and/or phosphatidylglycerol (PG) exposed at the surface of the bacterial membrane. Such electrostatic interactions, sufficient to cause the release of divalent cations, could explain the selectivity of CAAs for the bacterial membrane over the eukaryotic membrane mainly formed of neutral lipids at physiological pH (phosphatidylcholine and cholesterol) although negatively charged lipids and proteins are also represented (Salmi et al., 2008a, Savage et al., 2002).
Although required, the cationic charge is not sufficient per se to cause either the permeabilisation of bacterial membranes or the bacteria killing (Katsu et al., 2002, Savage et al., 2002). In fact, the hydrophobic moiety of CAAs is responsible for their insertion into the hydrophobic core of the bacterial lipid bilayer, leading to membrane permeabilisation and eventually to CAAs-mediated bacterial killing as convincingly indicated by the lack of membrane permeabilisation of a deacylated polymyxin analogue (Vaara and Vaara, 1983, Tsubery et al., 2000, Tsubery et al., 2001, Tsubery et al., 2002, Katz et al., 2003, Clausell et al., 2005, Clausell et al., 2007).
Based on similarities with their molecular target (i.e. LPS) and of their physicochemical properties (i.e. cationic detergent), one can roughly extrapolate that all CAAs interact in a similar way with biological membranes. However, this assumption is unlikely to be correct due to the structural diversity of their hydrophobic domain, the number of cationic groups and/or the variations in their polar:apolar ratio. Surprisingly, studies focusing on the comparison of the activities of various CAAs in the same membrane system are scarce (Ding et al., 2001, Xiong et al., 2005) although such studies are critical to understand the mechanism of action of CAAs on biological membranes and to develop more efficient analogues.
Among CAAs, squalamine (SQ), a steroid derivative initially isolated from dogfish shark (Moore et al., 1993), has attracted high interest for a broad range of researchers. SQ is currently used for the treatment of various human pathologies, such as macular degeneration or cancer-associated angiogenesis (Brunel et al., 2005). Moreover, SQ has been proposed as a potential new class of antimicrobial agents against multidrug resistant Gram-negative and Gram-positive bacteria (Moore et al., 1993, Ding et al., 2001, Savage and Li, 2000, Savage et al., 2002, Brunel et al., 2005, Salmi et al., 2008a, Salmi et al., 2008b).
Whereas the interaction of other CAAs with biological membranes has been the subject of numerous studies, particularly with polymyxin B (PMB) (Chen and Feingold, 1972, Clausell et al., 2004, Clausell et al., 2005, Clausell et al., 2007, Hartmann et al., 1978, Katsu et al., 1984, Katsu et al., 2002, Katz et al., 2003, Sixl and Galla, 1981, Sixl and Galla, 1982, Tsubery et al., 2000, Tsubery et al., 2001, Tsubery et al., 2002, Wiese et al., 1998, Wiese et al., 2003), relatively little information is available regarding the interaction of SQ with cellular membranes (Chen et al., 2006, Salmi et al., 2008a, Selinsky et al., 1998, Selinsky et al., 2000).
In the present study, we have investigated the interaction of SQ with prokaryotic and eukaryotic membranes (whole cells, lipid monolayers and lipid bilayers) and compared it with two other CAAs, i.e. the antimicrobial polymyxin B (PMB) and the cationic detergent hexadecyltrimethylammonium bromide (CTAB).
Our data shed some light on the mechanism of interaction of SQ with biological membranes and demonstrate that, although CAAs share common biophysical properties, they possess some specificity of interaction that depends on the nature of the biological membrane studied. Most importantly, our results suggest that the distribution coefficient (log D) could be a valuable indicator of the CAAs selectivity for bacteria over eukaryotic cells, a required condition to guarantee the innocuousness of an antimicrobial used in human/animal medicine.
Section snippets
Cell culture and bacteria
Escherichia coli (ATCC No. 54127) were maintained on Luria–Bertani (LB) agar plate at 37 °C. The day before experiments, single colonies were collected and grown over-night in LB solution at 37 °C. Bacterial number was then evaluated by measuring the optical density (OD) of the LB suspension at 600 nm (OD600 nm of 1 corresponding to approximately to 109 bacteria/mL as determined by colony counting after bacteria plating). Murine B lymphoma (Wehi-231, ATCC No. CRL-1702) were grown as cell suspension
Structure and biophysical properties of SQ, CTAB and PMB
We compared the structure and the biophysical properties of SQ, CTAB and PMB (Fig. 1 and Table 1). The three molecules have in common a charged polar domain and an apolar domain. However they differ in both the number of charges and the chemical structure of the apolar moiety. The hydrophobic domain of SQ (Fig. 1A), corresponds to a cholestane structure whereas the polar part consists of three cationic charges, due to the polyamine spermidine, and an anionic charge linked to a sulphate group,
Discussion
The cholesterol derivative squalamine, one of the members of CAAs, has been proposed as new drug for the treatment of macular degeneration and cancer and as a new class of antibiotics (Brunel et al., 2005, Salmi et al., 2008a, Savage and Li, 2000, Savage et al., 2002). However, the mechanism of interaction of SQ with prokaryotic and eukaryotic membranes remains mostly unknown. In particular, no comparison has been made between the interaction of SQ and other CAAs with biological membranes. In
Acknowledgments
We acknowledge Pr M. Zasloff from Georgetown University, Washington (USA) providing us squalamine. We thank Henri Chahinian and Nicolas Garmy for helpful discussions and stimulating advice throughout this study. We also thank Sabine Quitard for her help with the manuscript. This work has been supported by academic funds from University Paul Cezanne, the Centre National de la Recherche Scientifique (CNRS) and a grant (FAN-BCL) from the Institut National du Cancer (INCA).
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