Biological Activities of Lasso Peptides and Structure–Activity Relationships - Lasso Peptides: Bacterial Strategies to Make and Maintain Bioactive Entangled Scaffolds - page 42

The main antagonist described for NP receptors are analogues to this class of
hormones (von Geldern et al. 1990 ; Delporte et al. 1992 ; Cunningham et al. 1994 ;
Deschênes et al. 2005 ; Fig. 2.2b ). In addition, several non-peptidic inhibitors have
been proposed, such as the fungal polysaccharide HS-142-1 (Morishita et al. 1991 ;
Poirier et al. 2002 ), the indole derivative isatin (Glover et al. 1995 ) and the mono-
clonal antibody 3G12, the latter being specific to NPR-B (Drewett et al. 1995 ).
Given the positive effects of NPs, the interest of receptor antagonists resides mainly
in providing a tool to better understand the physiology of the natriuretic system. In
1991, Weber et al. showed that anantin (Fig. 3,2c ), a 17-residue peptide predicted
as having the lasso topology, but for which the three-dimensional structure has not
been published, binds competitively to ANP receptors from bovine adrenal cortex
and inhibits the intracellular cGMP accumulation in bovine aorta smooth muscle
cells, in a dose-dependent manner (Weber et al. 1991 ). This effect was measured
from competitive experiments in the presence of 125 I-labelled ANP on bovine ad-
renal cortex membranes. The IC 50 value was 1 µM, which is 4,000-fold less po-
tent than rat ANP (103-126), and the K d deduced from the competition curves was
0.61 µM. Des-phe-anantin, a side product of anantin missing the C-terminal Phe17,
was 50 times less potent than anantin. In 1993, Trachte reported that anantin had
no antagonist properties on the neuromodulatory effects of ANP, showing that this
activity does not rely on cGMP production (Trachte 1993 ). This effect was later
attributed to the receptor NPR-C (Trachte 2005 ). Therefore, anantin is recognized
as a selective antagonist of the guanylyl cyclase NP receptor of ANP, i.e. NPR-A.
Anantin has been used extensively as an antagonist to investigate the role and
molecular mechanisms of the natriuretic system (recent selection: Citarella et al.
2009 ; Abraham et al. 2010 ; Hrometz et al. 2011 ; Baetz et al. 2012 ; Bian et al. 2012 ;
Maeda et al. 2013 ; Vilotti et al. 2013 ) and is cited in more than 100 patents. How-
ever, most of the studies reported have used commercially available versions of
anantin, which are peptides obtained by solid-phase synthesis. Depending on the
providing company, these peptides are either linear or branched cyclic (i.e. with
the macrolactam ring), but they cannot display a lasso topology since this specific
fold has never been obtained by chemical synthesis, as described in Chaps. 2 and
4. Although the three-dimensional structure of anantin has not been resolved, it is
highly probable that it adopts a lasso structure. Therefore, the activities reported for
the linear and branched-cyclic variants cannot be attributed to anantin sensu stricto.
These synthetic peptides display antagonist activities on NPR-A. Therefore, the
lasso topology appears not to be a requisite for this activity. However, a comparative
study of the affinities of the lasso, branched-cyclic and linear variants of anantin for
NPR-A would be necessary to better understand the structure/activity relationships
of these peptides and use the most relevant form as an antagonist in the future.
The lasso peptide BI-32169, produced by Streptomyces sp., is a strong antagonist
of the glucagon receptor (Potterat et al. 2004 ; Knappe et al. 2010 ). Glucagon is a
29-amino acid peptide hyperglycaemic hormone. Its protein precursor progluca-
gon is encoded by a gene distributed in vertebrates and highly conserved within
mammals (Irwin 2001 ). In mammals, proglucagon is converted into three distinct
structurally related peptides, glucagon, glucagon-like peptide 1 (GLP-1) and glu-
cagon-like peptide 2 (GLP-2; Fig. 3.3a ). These peptides play essential roles in the