Allosteric inhibitors of plasma membrane Ca²⁺pumps: Invention and applications of caloxins

Abstract

Plasma membrane Ca(2+) pumps (PMCA) play a major role in Ca(2+) homeostasis and signaling by extruding cellular Ca(2+) with high affinity. PMCA isoforms are encoded by four genes which are expressed differentially in various cell types in normal and disease states. Therefore, PMCA isoform selective inhibitors would aid in delineating their role in physiology and pathophysiology. We are testing the hypothesis that extracellular domains of PMCA can be used as allosteric targets to obtain a novel class of PMCA-specific inhibitors termed caloxins. This review presents the concepts behind the invention of caloxins and our progress in this area. A section is also devoted to the applications of caloxins in literature. We anticipate that isoform-selective caloxins will aid in understanding PMCA physiology in health and disease. With strategies to develop therapeutics from bioactive peptides, caloxins may become clinically useful in cardiovascular diseases, neurological disorders, retinopathy, cancer and contraception.

A Selective PMCA Inhibitor Does Not Prolong the Electroolfactogram in Mouse

“…Much but not all of the evidence that indicates a role for PMCA in response recovery derives from the use of PMCA inhibitors [8], [10], [11], [13]. As noted elsewhere [14], [15], the traditional inhibitors, eosin and vanadate, inhibit not just PMCA but all ATPases, including the Na + , K + -ATPase. Na + , K + -ATPase is expressed in olfactory cilia [16], [17].…”

Plasma Membrane Calcium ATPases as Novel Candidates for Therapeutic Agent Development

“…A more promising strategy to develop PMCA-specific inhibitors was introduced by Ashok Grover, whose laboratory focused on isolating short (~10-15 residues) PMCA-binding peptides by panning of random peptide phage display libraries with specific PMCA sequences corresponding to the pump’s extracellular loop regions [127]. These so-called caloxins (for extracellular plasma membrane Ca 2+ pump inhibitors, in analogy to the Na + /K + pump inhibitor digoxin) act as allosteric modulators of the PMCA presumably by interfering with important conformational transitions that occur during normal pump function [128].…”

“…Caloxins against the first, second and third extracellular loop of the PMCA (see Fig. 1) have been developed and accordingly were named caloxins 1, 2, and 3, respectively (see [127] for explanation of the caloxin nomenclature). While first generation caloxins are not very isoform-specific and have relatively high K i values on the order of 0.5 mM (e.g., caloxin 2A1 [129]), the more recently developed caloxins (e.g., caloxin 1B1) show considerable isoform-selectivity and improved (lower) K i values (~50 μM) when added to the medium of intact cells [130].…”

“…The value of these more selective caloxins for whole-cell physiological studies of PMCA isoform function has been demonstrated in several recent publications, e.g., showing the importance of PMCA1 in endothelial cell Ca 2+ regulation [132] and of PMCA4 in aortic smooth muscle contraction [130, 131]. Caloxins with K i values in the low micromolar range are now available [127] and it is realistic to expect that further improvements in selection strategies and/or chemical modification will lead to the development of new generations of caloxins with even higher affinity and selectivity for specific PMCA isoforms.…”

Targeting apoptosis by the remodelling of calcium‐transporting proteins in cancerogenesis

“…Pande et al . suggested using caloxins, which are short peptides that specifically inhibit PMCA by binding to the allosteric sites on the protein's extracellular domains . Using caloxins, authors have been able to increase the calcium concentration in the cell and to promote the activation of apoptotic calcium‐dependent pathways.…”