Tài liệu Báo cáo khoa học: Structure-activity relationships of a-conotoxins targeting neuronal nicotinic acetylcholine receptors ppt

MINIREVIEW Structure-activity relationships of a-conotoxins targeting neuronal nicotinic acetylcholine receptors Emma L. Millard, Norelle L. Daly and David J. Craik Institute for Molecular Bioscience, University of Queensland, Brisbane, QLD, Australia a-Conotoxins that target the neuronal nicotinic acetylcho- line receptor have a range of potential therapeutic applica- tions and are valuable probes for examining receptor subtype selectivity. The three-dimensional structures of about half of the known neuronal specific a-conotoxins have now been determined and have a consensus fold containing a helical region braced by two conserved disulfide bonds. These disulfide bonds define the two-loop framework char- acteristic for a-conotoxins, CCX m CX n C, where loop 1 comprisesfourresidues(m¼ 4) and loop 2 between three and seven residues (n ¼ 3, 6 or 7). Structural studies, par- ticularly using NMR spectroscopy have provided an insight into the role and spatial location of residues implicated in receptor binding and biological activity. Keywords: NMR; peptide; X-ray crystallography. Introduction As outlined in other articles in this series, the a-conotoxins have a range of potential therapeutic applications and have proved to be valuable pharmacological tools based on their ability to selectively inhibit the nicotinic acetylcholine receptor (nAChR) [1–3]. The focus of this review is on the three-dimensional structures of a-conotoxins and the progress made towards dissecting the features involved in receptor subtype selectivity. In particular, a-conotoxins targeting neuronal rather than muscle nAChRs will be discussed. Muscle specific a-conotoxins have been covered in other more general reviews [4–6]. There is much current interest in various neuronal receptor subtypes implicated in diverse neurological disorders such as Alzheimer’s disease and epilepsy [7–9], and in the regulation of small-cell lung carcinoma [10,11]. The sequences, subtype selectivity and potency of a-conotoxins targeting neuronal nAChRs are given in Table 1, together with information on their structural characterization. The cysteine residues and disulfide con- nectivity are invariant throughout these sequences and define a two-loop framework, CCX m CX n C(X m and X n refer to the number of noncysteine residues), where the loops correspond to the residues between successive cysteine residues. The number of residues in the two loops (m/n) is used to group the a-conotoxins into different frameworks. ImI and ImII have a 4/3 framework and the other peptides in Table 1 contain either a 4/6 or 4/7 framework. It is interesting to note that although the majority of 4/6 and 4/7 a-conotoxins are selective for neuronal nAChRs, conotoxin EI contains a 4/7 framework but binds to the muscle-type nAChR [12]. The sequence conservation of the a-conotoxins extends beyond the cysteine residues, with a Ser and Pro in loop 1 being highly conserved. However, there is a significant degree of sequence variation in the remaining residues, particularly in loop 2. It is this sequence diversity that provides the exquisite selectivity that a-conotoxins display for various nAChR subtypes (Table 1). Structures of neuronally active a-conotoxins, in conjunction with activity studies, have provided clues to understanding the complexity involved in binding to the nAChR. A summary of this structural information and the insights into structure-activity relation- ships of a-conotoxins is presented in this review. Structural features of a-conotoxins The three-dimensional structures of a-conotoxins have been determined, primarily using NMR spectroscopy. It is unusual for such small peptides to crystallize but a few a-conotoxins have been amenable to analysis with X-ray crystallography. To date no neuronally active conotoxins have been structurally characterized using both techniques, however, the neuromuscularly active conotoxin GI has been studied using both methods and the structures overlay very closely [13–15]. Despite the small size of a-conotoxins they have well- defined structures with a characteristic overall fold. With the structures of more than half of the known neuronally active a-conotoxins determined it is possible to determine the consensus structural features. These features involve restraints imposed by the conserved disulfide connectivity and a helical region centred around Cys III. The helix typically encompasses residues 5–12. A comparison of the known structures is given in Fig. 1 with the three framework classes presented separately for clarity. It is clear that the backbone fold of loop 1 is highly conserved, including the first turn of the helix. The major differences, as might be Correspondence to D. Craik, Institute for Molecular Bioscience, University of Queensland, Brisbane, QLD, 4072, Australia. Fax: + 61 73346 2029, Tel.: + 61 73346 2019, E-mail: d.craik@imb.uq.edu.au Abbreviation: nAChR, nicotinic acetylcholine receptor. (Received 22 January 2004, revised 19 March 2004, accepted 6 April 2004) Eur. J. Biochem. 271, 2320–2326 (2004) Ó FEBS 2004 doi:10.1111/j.1432-1033.2004.04148.x expected, occur in loop 2, a direct reflection of the differing number of residues seen in this loop. However, even when the residue numbers are the same, as in the overlay of 4/7 a-conotoxins, loop 1 superimposes better than loop 2. In this case the structural differences are related to the sequence diversity rather than the number of residues. Analysis of the surface features of a-conotoxins reveals the fact that the cysteine residues are significantly more buried than the other residues, although the small size of the molecules prevents complete burial. As a consequence of this burial, a small surface exposed hydrophobic patch is present in the a-conotoxins [16]. The extent of this hydrophobic patch varies amongst the different conotoxins but generally involves residues adjacent or close in sequence to Cys III. The charge distribution also varies amongst the a-conotoxins and this may influence the differences observed in potency and specificity. Indeed, it was originally thought that the net overall charge was related to the nAChR specificity [17], with a-conotoxins that target muscle subtypes having net positive charges and those targeting neuronal being either negative or neutral. ImI is an exception [18] as is the recently discovered ImII [19]. The overall quality of the structures determined is very good and appears to be not only related to the compact disulfide connectivities but also to a network of hydrogen bonds. As expected, an analysis of the structures reveals that hydrogen bonds are associated with the a-helical region, but other parts of the structure also contain hydrogen bonds. Several structures have been determined independently by different groups and the structures appear to be in good agreement. The structure of ImI in solution has been determined in five studies, all in aqueous solution [18,20–23]. They are all similar and show the basic backbone fold seen in all a-conotoxins. These structures were determined at various pH values between 3.0 and 6.0 and all were of high precision, with backbone rmsd values between 0.34 A ˚ and 0.78 A ˚ . Two different structures of MII have also been published. One of the structures was determined in aqueous buffer at pH 3.3 [24] and the second was determined in aqueous solution (pH 3.9) as well as in 30% trifluoro- ethanol/H 2 O and 30% acetonitrile/H 2 O [25]. Both struc- tures showed the same fold and were well-defined with backbone rmsd values of 0.76 and 0.07 A ˚ , respectively. Two reports also exist for the structure of AuIB [26,27]. Both were determined in aqueous solution with similar pH values and backbone rmsd values of 0.27 and 0.36 A ˚ . The second study extended the investigation to examine the role of different disulfide bond isomers in determining structures. The structures of PnIA, PnIB and the desulfated form of native EpI ([Y15]EpI) were determined using X-ray crys- tallography [17,28,29]. Recently discovered a-conotoxins, ImII [19], GID [16], AnIB [30] and PIA [31], demonstrate the fact that the diversity of conotoxin primary structures will probably increase as more are discovered. In ImII, a highly conserved proline residue present in all other a-conotoxins is not present, while in GID, AnIB and PIA an N-terminal extension, or tail, not seen in any other a-conotoxin is present. A novel a-conotoxin (Vc1.1) that displays signifi- cant sequence variation in loop 2 compared to previously characterized a-conotoxins has also recently been identified by gene sequencing of Conus victoriae [32]. The absence of Pro6 in ImII may have implications for its mechanism of action as this residue is thought to be important for activity in ImI. This is supported by the fact that although ImII is still active at the a7 nAChR, it appears to act at a different binding site from ImI [19]. The structure of ImII has not yet been determined, but may provide further information on the importance of the proline substitution. GID incorporates an N-terminal tail that contains four residues prior to the first cysteine residue [16]. This is the largest of the a-conotoxins reported to date. GID also contains post-translational modifications not previously reported for a-conotoxins. A c-carboxyglutamic acid is present at position 4 and a hydroxyproline at position 16. Both of these modifications are common in other classes of conotoxins [33–39]. Interestingly, the post-translational modification most commonly found in a-conotoxins, namely an amidated C-terminus, is not present in GID. Table 1. Sequence, receptor specificity and structural information on neuronally active a-conotoxins. *Refers to the amidated C-terminus. IC 50 relates to receptors expressed in Xenopus oocytes. rmsd/resolution: rmsd is relevant for the NMR structures, resolution refers to those structures completed by X-ray crystallography. The cysteine residues are highlighted in bold. Conotoxin Sequence m/n nAChR subtype IC 50 (n M ) Method rmsd/resolution (A ˚ ) MII GCCSNPVCHLEHSNLC* 4/7 a3b2 a 0.5 [54] NMR 0.07 PnIA GCCSLPPCAANNPDYC* 4/7 a7, a3b2 a 252, 9.6 [47] X-ray 1.1 PnIB GCCSLPPCALSNPDYC* 4/7 a7, a3b2 a 61.3, 1970 [47] X-ray 1.1 EpI GCCSDPRCNMNNPDYC* 4/7 a3b2, a3b4 a 1.6 c [55] X-ray 1.1 GIC GCCSHPACAGNNQHIC* 4/7 a3b2 b 1.1 [51] – – GID IRDcCCSNPACRVNNOHVC 4/7 a7, a3b2, a4b2 a 5, 3, 150 [16] NMR 0.34 PIA RDPCCSNPVCTVHNPQIC* 4/7 a6b2b3, a3b2 a 0.95, 74.2 [31] – – AnIB GGCCSHPACAANNQDYC* 4/7 a3b2, a7 a 0.3, 76 [30] – – AuIA GCCSYPPCFATNSDYC* 4/6 a3b4 a >750 [56] – – AuIC GCCSYPPCFATNSGYC* 4/6 a3b4 a >750 [56] – – AuIB GCCSYPPCFATNPD-C* 4/6 a3b4 a 750 [56] NMR 0.27 ImI GCCSDPRCAWR C* 4/3 a7 a 132 [19] NMR 0.34 ImII ACCSDRRCRWR C* 4/3 a7 a 441 [19] – – a rat nAChR subunits, b human nAChR subunits, c ACh-evoked currents in parasympathetic neurons from rat intracardiac ganglia. Ó FEBS 2004 Structures of neuronally active a-conotoxins (Eur. J. Biochem. 271) 2321 Structure-activity relationships of a-conotoxins Although there is a strong conservation of certain residues in a-conotoxins, the natural sequence variation has enabled the conotoxins to be used as pharmacological tools in helping us to understand the specificity of both neuronal and neuromuscular nAChRs. Mutational analyses have also been valuable in elucidating factors important for activity. Table 2 lists the most informative mutations that have been made in a range of a-conotoxins. In addition, an alanine scan of the non-cysteine residues has been performed on PnIB [38]. Extensive mutational analysis has been carried out on ImI, PnIA, PnIB and to a lesser extent on GID and MII. Analysis of ImI has revealed that Asp5-Pro6-Arg7 and Trp10 are important for biological activity [40–43] at the neuronal a7 nACh receptor. Many studies have involved residue substitution with Ala, however, replacements with other residues have allowed fine details to be discerned. For example, substitution of Trp10 with Tyr or Phe had little or no effect on the binding of ImI, indicating that an aromatic residue was required at this position for activity [42–45]. Further point mutations of ImI were performed by Rogers et al. ([R11E]ImI, [R7L]ImI and [D5N]ImI) and Lamthanh et al. ([R7A]ImI) and the three-dimensional structures of the mutants were determined [21,46]. It was noted that very small conformational changes in ImI, especially for the side chains involved in binding, are associated with a loss of activity. Analysis of the molecular surfaces reveals that the side chains of the active residues in ImI are on a solvent accessible face of the molecule. Figure 2 shows surface representations of ImI, detailing the position of the residues Fig. 1. Consensus structural features of neuronally active a-conotoxins. (A) On the left is an overlay of the three-dimensional structures of the 4/7 framework a-conotoxins, PnIA (dark blue), PnIB (green), EpI (light blue), MII (red) and GID (gold) superimposed over residues 3–10 for all except GID, which was superimposed over the corresponding residues 6–13. This comparison highlights the conservation of the helical region in these peptides and the variation observed in loop 2. The backbone atoms are shown in stick format and the N- and C-termini are labeled. A schematic representation of the conserved residues in the 4/7 a-conotoxin sequences is shown on the right. Disulfide connectivities are shown by connections between the conserved cysteine residues and amino acids that are conserved throughout most of the neuronally active a-conotoxins are indicated by blue shaded circles and single letter amino acid codes. Red circles indicate the residues that have been associated with biological activity in one or more conotoxin. White circles represent regions in the molecules where there is variability in the residue type and in the case of loop 2 in both the type and number of residues. The N- and C-termini are labeled. (B) On the left is the three-dimensional structure of the 4/6 framework a-conotoxin, AuIB shown in stick format with the N- and C-termini labeled. On the right is the corresponding schematic representation of the conserved residues in the 4/6 a-conotoxin sequences. (C) On the left is the three-dimensional structure of the 4/3 framework a-conotoxin, ImI shown in stick format. On the right is the corresponding schematic representation of the conserved residues in the 4/3 a-conotoxin sequences. Some conserved residues have been reported to be associated with biological activity and these are represented with blue outlined red circles. 2322 E. L. Millard et al.(Eur. J. Biochem. 271) Ó FEBS 2004 identified as important for biological activity at the a7 neuronal nAChR. It has been proposed that Asp5 and Pro6 contribute to receptor binding because of their structural role rather than through direct interaction at the binding site [46]. This is based on the empirical preference for Asp residues to be in the N-cap position of helices and the tendency for Pro to also be near the capping position. By capping (or initiating) the helical element these residues may play a vital role in correctly orienting key binding residues. Arg7 was suggested to have a functional role in binding as the structure of R7A only differs from the native in the side chain position of residue 7 but the analogue is not active [21,46]. Mutagenesis studies on PnIA have shown that substitu- tion of residues 10 and 11 has significant effects on the binding affinity of PnIA for receptors on native tissues [29,47–49]. The point mutations performed were [A10L]PnIA and [N11S]PnIA. These substitutions were chosen as they represent the residues that differ between PnIA and PnIB. This study showed that [A10L]PnIA had increased affinity for the a7 receptor and [N11S]PnIA has affinity decreased by  30-fold. The increase in potency of Table 2. Sequence and activity data for modified neuronally active a-conotoxins. D Refers to the truncated residues from the N-terminal tail of GID, L refers to lipoamino acid (Laa), the residues in blue are the mutated residues, the cysteines involved in disulfide bonds are in red, * refers to the amidated C-terminus. m/n Conotoxin Sequence nAChR subtype IC 50 relative to Native (n M ) Structural comparisons with native structures 4/3 [W10Y]ImI GCCSDPRCAYR C* a7 a no significant change [43] – [W10F]ImI GCCSDPRCAFR C* a7 a no significant change [42] – [R11E]ImI GCCSDPRCAWE C* a7 b no significant change [46] similar [R7L]ImI GCCSDPLCAWR C* a7 b fl [46] similar [D5N]ImI GCCSNPRCAWR C* a7 b fl [46] similar [R7A]ImI GCCSDPACAWR C* a7 c fl [21] similar Monodisulfide ImI GCCSDPRCAWR C* a7 c no significant change [21] relatively disordered 4/6 AuIB ribbon GCCSYPPCFATNPD-C* a3b4 d › [27] disordered 4/7 [A10L]PnIA GCCSLPPCALNNPDYC* a7, a3b2 a ›, fl [47] – [N11S]PnIA GCCSLPPCAASNPDYC* a7, a3b2 a fl [47] – [R12A]GID IRDc CCSNPACAVNNOHVC a7, a3b2, a4b2 a fl [16] – [D1–4]GID CCSNPACRVNNOHVC a7, a3b2, a4b2 a fl a4b2, no change for others [16] – LaaMII LGCCSNPVCHLEHSNLC* a3b2 d no significant change [53] minimal change from native MII 5LaaMII GCCSLPVCHLEHSNLC* a3b2 d fl [53] a-helix disrupted a rat nAChR subunits, b human nAChR subunits, c HEK cells used in competitive binding assay, d ACh-evoked currents in parasympathetic neurons from rat intracardiac ganglia. Fig. 2. Surface representations of ImI and PnIA. (A) The three-dimensional structure of ImI. The heavy atoms are shown in stick for- mat and the residues reported as important for a7-subtype activity at the neuronal nAChR, i.e.D5,P6,R7andW10,areshowninpink. The N- and C-termini are labeled. The two views are rotated by 180° about the y-axis. (B) The surface diagram of the three-dimensional structure of PnIA. The two residues reported to have significant influence on specificity and potency, A10 and N11, are shown in pink and labeled. Residues 4–7 and 9–10 in the closely related PnIB have also been shown to influ- ence activity [38]. Ó FEBS 2004 Structures of neuronally active a-conotoxins (Eur. J. Biochem. 271) 2323 [A10L]PnIAis thought to be associated with a slower rate of dissociation of the conotoxin from the a7 receptor [50]. The mutant studies carried out on PnIA suggest that both positions 10 and 11 have a significant influence on selectivity for the a7 subunit of the nAChR. NMR chemical shift analysis of the mutants provides a very sensitive method of assessing structural differences and in this case clearly shows that the changes in potency and selectivity are not related to structural changes in the backbone. A surface diagram of PnIA showing the residues important for binding is shown in Fig. 2. The increase in potency for [A10L]PnIA has been attributed to the longer side chain of the leucine at position 10. In general, for the 4/7 a-conotoxins there appears to be a correlation between the length of the aliphatic side chain in position 10 (numbering based on PnIA) and greater a7vs.a3b2 selectivity. As the length of the side chain at position 10 increases the conotoxins become more a7-selective and less a3b2-selective [16]. GIC has the smallest side chain possible, a glycine, and is a potent a3b2 inhibitor [51] although its selectivity with respect to a7activityhasyettobe determined. As the side chain length increases the ratio between a3b2anda7 increases up to > 100 for EpI containing a methionine at this position. The recent discovery of GID has added to knowledge of the structure-activity relationships of a-conotoxins by revealing that the highly charged N-terminal tail contributes to a4b2 activity. This was determined by analysis of a truncated analogue of GID ([D1–4]GID) that displayed no significant change in activity for the a7anda3b2 receptor subtypes [16], but a4b2 activity was significantly decreased. A predefined structure of this tail region is not present in solution and the three-dimensional structures indicate that this region is disordered. However, a particular conforma- tion may be present upon binding to the receptor. Further- more, the uncommon feature of an arginine residue at position 12 (Table 1) appears to contribute to a4b2anda7 subtype activity but not a3b2 activity. A decrease in a4b2 and a7 subtype activity but not a3b2 activity was observed in the [R12A]GID mutant [16]. An alternative approach to developing structure-activity relationships of a-conotoxins has been to examine the effects of re-engineering the disulfide bonds. A minimal scaffold has been found for ImI in which the Cys3 to Cys12 disulfide bond has been deleted [21]. Loss of this single disulfide bond had no effect on the binding affinity, and the overall structure is quite similar to the native although the peptide appeared to be more flexible than the native form. The structure was less well defined with an overall backbone rmsd value of 1.49 compared to 0.78 A ˚ for native ImI. Molecules with non-native disulfide connections have also been produced. This approach was first demonstrated for the muscle specific conotoxin GI, where all three possible disulfide bond isomers (globular, ribbon and beads, Fig. 3) were studied [52]. In this case the non-native (ribbon and beads) forms were less active and more flexible than the native globular isomer. The ribbon connectivity of AuIB has been synthesized and although the overall structure appears to be more disordered the biological activity was unexpectedly increased [27]. The structures of the native globular AuIB and ribbon AuIB are shown in Fig. 3, where the disruption in the helix is apparent in the latter. Given the importance of this element of structure in all of the native a-conotoxins it seems surprising that increased activity is observed when it is disrupted. However this may be rationalized by the fact that the ribbon form is more flexible than the native isomer and this may allow the molecule to better complement the binding surface of the receptor. However, the degree of flexibility is clearly important as too much flexibility leads to entropic losses in binding energy and potentially to decreased activity, as was noted for the non-native isomers of a-conotoxin GI. In common with other peptides, conotoxins have limita- tions on their use as therapeutic agents as they have poor bioavailability. Structural studies have provided insight into approaches aimed at improving the bioavailability of conotoxins. In particular, two lipophilic analogues of the conotoxin MII were recently developed [53], by adding a lipidic group (2-amino- D , L -dodecanoic acid, Laa) to either the N-terminus (LaaMII) or to Asn5 (5LaaMII). The N-terminal LaaMII was shown to have a tertiary structure similar to that of the native conotoxin and maintained the activity for the a3b2 subtype activity associated with the native peptide. However the 5LaaMII peptide did not adopt the helical structure seen in all the a-conotoxins and did not show any activity. This indicates a greater tolerance for modification at the N-terminal of MII than at residue 5. The active LaaMII was found to have improved permeability across Caco-2 cell monolayers compared to native MII and thus is considered to have potential for further in vivo biodistribution experiments [53]. Concluding remarks A conserved framework is evident in the three-dimensional structures of a-conotoxins, with the major element of secondary structure being an a-helix. The fold is largely determined by the conserved disulfide connectivity between Fig. 3. Disulfide bond isomers of a-conotoxins. (A) A schematic rep- resentation of the globular, ribbon and beads isomers possible in any of the a-conotoxins. (B) The three-dimensional structure of native AuIB with the globular disulfide connectivity, CysI-CysIII and CysII- CysIV. The disulfide bonds are shown in blue. (C) A three-dimensional structure of the ribbon disulfide bond isomer of AuIB, where the connectivities are CysI-CysIV and CysII-CysIII, with the disulfide bonds shown in blue. 2324 E. L. Millard et al.(Eur. J. Biochem. 271) Ó FEBS 2004 CysI-CysIII and CysII-CysIV that braces the structure. Altering the disulfide bonds has highlighted their important structural influence, as less defined structures are obtained when the connectivity is altered or when a single disulfide bond is removed. Interestingly, studies of non-native disulfide-bonded forms have also indicated that structural flexibility can influence the biological effects observed for a-conotoxins, either in a positive or negative way. Muta- tional analysis has indicated residues that are important for selectivity and potency, and structural analyses of such mutants have suggested that what appear to be only minor changes in the overall fold can have dramatic effects on receptor activity. 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