Mu-conotoxins as leads in the development of new analgesics

Abstract

Voltage-gated sodium channels (VGSCs) contain a specific binding site for a family of cone shell toxins known as mu-conotoxins. As some VGSCs are involved in pain perception and mu-conotoxins are able to block these channels, mu-conotoxins show considerable potential as analgesics. Recent studies have advanced our understanding of the three-dimensional structures and structure-function relationships of the mu-conotoxins, including their interaction with VGSCs. Truncated peptide analogues of the native toxins have been created in which secondary structure elements are stabilized by non-native linkers such as lactam bridges. Ultimately, it would be desirable to capture the favourable analgesic properties of the native toxins, in particular their potency and channel sub-type selectivity, in non-peptide mimetics. Such mimetics would constitute lead compounds in the development of new therapeutics for the treatment of pain.

Figure 1

Amino acid sequences of μ-conotoxins from the (a) M-4, and (b) M-5 branches of the M-superfamily [15,16]. Horizontal lines denote the location of disulfide bridges. Z represents pyroglutamate, O trans-4-hydroxyproline, and * a C-terminal amide. References for the amino acid sequences are as follows: μ-GIIIA, μ-GIIIB and μ-GIIIC [17], μ-TIIIA [18], μ-PIIIA [19], μ-SxIIIA and μ-SxIIIB [20]; μ-KIIIA and μ-SIIIA [21], μ-SIIIB [22], μ-SmIIIA [23], μ-BuIIIA and μ-BuIIIB [24], μ-CIIIA, μ-CnIIIA, μ-CnIIIB and μ-MIIIA [25]. The sequence of μ-BuIIIC [24] is not shown in this Figure; it contains eight residues in the first loop. The disulfide connectivities shown above the sequences in (b) have been confirmed experimentally for a number of the M-4 and M-5 toxins shown here, but not yet for all.

Figure 2

Stereo views of families of 20 final structures of μ-KIIIA (blue) [BMRB accession number 20048] [40], μ-SIIIA (orange) [BMRB accession number 20023] [41] and μ-SmIIIA (purple) [PDB ID 1Q2J] [42], superimposed over backbone heavy atoms (N, C^α, C’) of residues 4-16 for μ-KIIIA (residues 8-20 for μ-SIIIA and residues 10-22 for μ-SmIIIA).

Figure 3

Surface representations of the structures of μ-KIIIA [BMRB accession number 20048] [40], μ-SIIIA [BMRB accession number 20023] [41] and μ-GIIIA [PDB ID 1TCG] [43]. Arg residues are coloured dark blue, Lys cyan, Asp red, Cys yellow, Trp purple and all others grey. The lower views are flipped by 180º around the vertical axis relative to the upper views. Structural figures were prepared using PyMOL (Delano, W.L. The PyMOL Molecular Graphics System, Delano Scientific, San Carlos, CA, USA; 2002 http://www.pymol.org).

Figure 4

(a) Left hand side: Schematic of i to i + 4 lactam bridge linking Asp and Lys side chains. Right hand side: model of helical peptide containing this lactam bridge. (b) Schematics of peptides containing β2 (left) and β3 (right) amino acids.

Figure 5

Schematic of a helical mimetic scaffold. This shows a benzoylurea scaffold decorated with three substituents, R¹, R² and R³, which could mimic key functional side chains from the helical region of a µ-conotoxin such as µ-KIIIA or µ-SIIIA. X and Y can be any substituent, including additional aromatic rings.