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Effects of Antidepressants on the Conformation of Phospholipid Headgroups Studied by Solid-State NMR

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Effects of Antidepressants on the Conformation of Phospholipid Headgroups Studied by Solid-State NMR MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 2004; 42: 105–114 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/mrc.1327 Effects of antidepressants on the conformation of phospholipid headgroups studied by solid-state NMR Jose S. Santos,1† Dong-Kuk Lee1,2 and Ayyalusamy Ramamoorthy1,2,3∗ 1 Biophysics Research Division, University of Michigan, Ann Arbor, Michigan 48109-1055, USA 2 Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, USA 3 Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109-1055, USA Received 10 June 2003; Revised 25 August 2003; Accepted 1 September 2003 The effect of tricyclic antidepressants (TCA) on phospholipid bilayer structure and dynamics was studied to provide insight into the mechanism of TCA-induced intracellular accumulation of lipids (known as lipidosis). Specifically we asked if the lipid–TCA interaction was TCA or lipid specific and if such physical interactions could contribute to lipidosis. These interactions were probed in multilamellar vesicles and mechanically oriented bilayers of mixed phosphatidylcholine–phosphatidylglycerol (PC–PG) phospholipids using 31Pand14N solid-state NMR techniques. Changes in bilayer architecture in the presence of TCAs were observed to be dependent on the TCA’s effective charge and steric constraints. The results further show that desipramine and imipramine evoke distinguishable changes on the membrane surface, particularly on the headgroup order, conformation and dynamics of phospholipids. Desipramine increases the disorder of the choline site at the phosphatidylcholine headgroup while leaving the conformation and dynamics of the phosphate region largely unchanged. Incorporation of imipramine changes both lipid headgroup conformation and dynamics. Our results suggest that a correlation between TCA-induced changes in bilayer architecture and the ability of these compounds to induce lipidosis is, however, not straightforward as imipramine was shown to induce more dramatic changes in bilayer conformation and dynamics than desipramine. The use of 14N as a probe was instrumental in arriving at the presented conclusions. Copyright 2004 John Wiley & Sons, Ltd. KEYWORDS: NMR; 14NNMR;31P NMR; desipramine; imipramine; iminodibenzyl; phospholipid; antidepressant; tricyclic antidepressants INTRODUCTION renders the lipid unsuitable for degradation, or with the phospholipase thereby inactivating the enzyme.11 Since the The class of tricyclic antidepressants, which includes process of sequestering peripheral membrane and cytosolic imipramine and its metabolized product desipramine proteins to the membrane has been observed to be initially (Fig. 1), is known to interact strongly with the lipid bilayer.1–5 driven by an electrostatic interaction between the lipid head- The ‘amine hypothesis’ suggests that the pharmacological groups and protein,15–18 it is plausible that the introduction effect of these drugs is to block the re-uptake of neuro- of the charged TCA drug changes the electrostatic profile of transmitters at the nerve synapses.6 It is also known that the lipid bilayer and therefore may contribute to inhibit the the incorporation of these compounds into lipid bilayers phospholipase–membrane contact. In this conceptual frame, leads to intracellular accumulation of various phospholipids, it is important to ask how the lipid bilayer structure and a process known as lipidosis.7–12 Tricyclic antidepressant dynamics change at the atomic level upon the introduction (TCA)-induced lipidosis has been linked to the ability of these of positively charged amphiphiles. Specifically, it would be compounds to inhibit phospholipases which are responsible valuable to test if there is a correlation between the different for phospholipid degradation.6,13,14 However, the mecha- observed physiological effects of TCA homologues, such as nistic connection between TCA–lipid interaction and phos- desipramine and imipramine, and spectroscopically observ- pholipidosis remains puzzling. It is unclear whether a TCA able changes in the electrostatic profile of the membrane interacts directly with the phospholipids and consequently surface. The introduction of any type of inclusion, here defined Ł Correspondence to: Ayyalusamy Ramamoorthy, Biophysics as any non-lipid molecule, into a lipid bilayer rearranges Research Division, University of Michigan, Ann Arbor, Michigan 48109-1055, USA. E-mail: [email protected] the number and type of interactions that stabilize the bilayer †Present address: Division of Biological Sciences, Section of structure. Probing these types of interactions in a quanti- Neurobiology, University of California at San Diego, La Jolla, tative manner has proven difficult through spectroscopic California 92093-0366, USA. Contract/grant sponsor: National Science Foundation. methods. The work of Pasenkiewicz-Gierula and co-workers Contract/grant sponsor: Petroleum Research Fund. using molecular simulations of lipid bilayers to investigate Copyright 2004 John Wiley & Sons, Ltd. 106 J. S. Santos, D.-K. Lee and A. Ramamoorthy O22 H C218 C216 C214 C212 CC28 C26 C24 C22 A N C217 C215 C213 C211 C29 C27 C25 C23 C21 O21 O13 C2 O11 O C A 12 12 C13 C 1 P C11 H3C C H3C 3 + + N BCHHN + H N CH O O γ 3 31 14 C14 C C34 C32 C15 C C C C38 36 C316 314 312 310 C β C C C33 31 α C C39 37 35 C315 C313 311 6 4 O 3 N 7 32 5 N 8 2 O22 C218 C216 C214 C212 CC28 C26 C24 C22 1 9 C217 C215 C213 C211 C29 C27 C25 C23 C21 11 10 H O21 Figure 1. Structures of TCA drugs: (A) iminodibenzyl; O13 O14 (B) desipramine; (C) imipramine. C O 2 11 O12 C12 B C 1 P C11 C13 C3 O15 these types of forces has recently produced a wealth of O31 O H 19,20 14 information. These studies showed that there are two C C34 C32 C C38 36 C316 C314 C312 310 C C C types of short-distance interactions that are predicted to C C37 35 33 31 C C313 C311 39 occur between phosphatidylcholine (PC) headgroups. These 315 O32 interactions are water-mediated hydrogen bonds and charge association (charge pairs), each of which can be inter- or Figure 2. Structures of (A) 1-palmitoyl-2-oleoyl-sn-glycero- intramolecular.19,21,22 Such interactions are responsible for 3-phosphatidylcholine (POPC) and (B) 1-palmitoyl-2-oleoyl-sn- the formation of an extended network among PC head- glycero-3-phosphatidylglycerol (POPG). The phospholipid groups linking 98% PC in a membrane.19 The sites of these atom numbering scheme is identical with that used by 19 interactions are the positively charged nitrogen atom in the Pasenkiewicz-Gierula et al. choline headgroup, the negatively charged non-ester phos- phate oxygens (see O13 in Fig. 2) and the carbonyl oxygens In this study, the interaction of TCAs with mechani- (see O22 and O32 in Fig. 2). cally oriented lipid bilayers and multilamellar lipid vesicles Solid-state NMR techniques on naturally abundant NMR- composed of a mixture of zwitterionic (PC) and anionic active nuclei of 31Pand14N (100.0 and 99.63%, respectively) (phosphatidylglycerol) lipids was monitored by solid-state are ideal to study these changes in the environment near NMR techniques using as probes the 31Pand14NNMR the lipid headgroup in model lipid bilayers. Nitrogen-14 is active nuclei located at the lipid headgroups. In general, it a quadrupole nucleus that shares deuterium’s sensitivity to is shown that 14NNMRmaybecoupledto31PNMRtech- changes in bilayer surface charge (the ‘molecular voltmeter niques to distinguish between changes in lipid headgroup effect’).23 In spite of its high natural abundance and sensitiv- conformation and dynamics upon the inclusion of different ity to changes in electric field gradient, relatively few studies charged amphiphiles. Specifically, the results show that the have made use of this nucleus.1,24–32 From the results of these inclusion of desipramine in lipid bilayers results primarily studies, three important conclusions of particular relevance in a decrease in order of the PC lipid headgroup while leav- to this investigation can be drawn. First, the 14N nucleus ing the conformation of the headgroup largely unchanged. appears to be more sensitive than deuterium since slight dis- In contrast, imipramine is shown to alter both conformation tortions of the tetrahedral symmetry upon addition of anionic and order of the lipid headgroups. Changes in overall bilayer amphiphiles result in large spectral changes.32 Second, the architecture were determined to be highly dependent on the order of the choline headgroup is independent of the order TCA’s effective charge and steric constraints. It is concluded of the acyl chains and the phosphate moiety, as shown by that the ability of TCAs to induce lipidosis in vivo is not the observed concurrent 28% decrease in the 14N quadrupole correlated in a straightforward manner with TCA-induced splitting and 20% increase in 31P chemical shift anisotropy changes in overall bilayer architecture. (CSA) span upon addition of the local anesthetic tetracaine (at a lipid to tetracaine molar ratio of 2.6 : 1).30 Finally, the EXPERIMENTAL choline headgroup is sensitive to desipramine, as evidenced by the 50% decrease in the lipid quadrupole splitting in the Materials presence of the drug in a 1 : 2 mole ratio.1 Nitrogen-14 has an The phospholipids, 1-Palmitoyl-2-oleoyl-sn-glycero-3-phos- additional advantage in studying model lipid bilayers as it phatidylcholine (POPC) and 1-palmitoyl-2-oleoyl-sn-gly- allows for the isolated study of PC since most other 14Nnuclei cero-3-phosphatidylglycerol (POPG) were purchased from (from proteins, other lipids or drugs) do not contribute to the Avanti Polar Lipids (Alabaster, AL, USA) and used without 14N NMR signal due to large quadrupole coupling constants. further purification. The purity of the lipids was checked Copyright 2004 John Wiley & Sons, Ltd. Magn. Reson. Chem. 2004; 42: 105–114 Effects of antidepressants on the conformation of phospholipid headgroups 107 ° µ by thin-layer chromatography using CH3Cl–CH3OH–H2O the 90 pulse width and delay were 3–4 and 80 s, respec- (85 : 30 : 3) as solvent system and iodine as visualiza- tively.
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