Molecular targets of nephrotoxic action of polymyxins

Анотація

The rapid spread of multidrug-resistant gram-negative bacterial strains has necessitated the search for the more efficient antimicrobial agents and prompted a renewed interest in polymyxins which have been invaluable for the therapy of serious nosocomial pathogens but withdrawn due to their nephrotoxicity. Polymyxins are nonribosomal cyclic lipopeptides isolated from Paenibacillus polymyxa. Several distinct groups of polymyxins have been structurally identified, each group is characterized by the unique amino acid sequence and the chemical structure of the fatty acyl group. But, only two polymyxins, polymyxin B and colistin (polymyxin E), have been clinically used. The revival of polymyxins into clinical practice has stimulated further thorough investigations of their toxicity. During the last decade, the toxicity of polymyxin B and colistin has been thoroughly studied (taking into account the chemical purity, homogeneity, dosing regimens and other factors) and appeared to be not as high as reported earlier. Nevertheless, it still may substantially complicate therapy and even result in its stoppage. Thus, the development of less toxic polymyxin derivatives remains to be a topical problem. Understanding of molecular mechanism(s) of polymyxin’s toxicity based on detailed knowledge of the peculiarities of their intermolecular interactions with the targets of their toxic action is a prerequisite of a purposeful search for such polymyxin-based compounds. Nephrotoxic effect of polymyxins is determined by their accumulation in the epithelial cells of the kidney proximal tubules. The main factor of the accumulation of these antibiotics considered to be their interaction with megalin, the giant receptor of cell surface, which is the most abundant in the apical membrane of renal proximal tubules. Megalin is a representative of the low-density lipoprotein receptor (LDL-receptor) family which contains several structurally homologous receptors. Megalin is the largest member of the family; molecular weight of rat megalin is about 600 кDа. Its amino acid sequence involves about 4660 amino acid residues and is identical to one of human megalin by approximately 77%. All members of this receptor family have a modular structure, in particular, they contain clusters of two or more cysteine-rich complement-type repeats (CR modules) which are the binding sites of the most ligands of LDL-receptors. Each of CR domains consists of approximately 40 amino acid residues. The binding site of the cationic ligands on the LDL-receptors shown to contain a common structure motif, so called DXDXD motif, which consists of three negatively charged aspartic acid residues coordinated by Са2+ ion and a hydrophobic residue. As a rule, the cationic part of a ligand is represented by lysine residue. The binding occurs mainly due to electrostatic interactions between positively charged lysine residue and negatively charged residues of aspartic acid. The binding is enhanced by hydrophobic interactions between aromatic residue of CR module and aliphatic region of lysine. Structural data on megalin are rather limited now: there is known a structure of CR12 domain of rat megalin and a structure of CR10 domain of human megalin (as well as its complexes with gentamicin) both solved by NMR in solution. Polymyxin binding site on the megalin is not yet determined experimentally, there are absent as well any structural models at atomic level for polymyxin interaction with megalin. However, based on an analysis of available data on the structure of ligand-target complexes for the members of LDL-receptors family, there are reasons to suppose that polymyxin’s molecular fragment which binds to megalin is represented by cationic residues of di-amino-butanoic acid (an analogue of lysine), and the structural DXDXD motifs of CR domains of megalin compose binding sites for polymyxin as well. In the review, the available data on the structure and three-dimensional organization of megalin and other members of LDL-receptors family are represented; the peculiarities of their ligand-target intermolecular interactions are considered. It is concluded that the weakening of polymyxins binding with megalin may be an effective preventive measure against polymyxin-induced nephrotoxicity.