Selective targeting of α7 nicotinic acetylcholine receptor by synthetic peptide mimicking loop I of human SLURP-1 provides efficient and prolonged therapy of epidermoid carcinoma in vivo

Non-conventional toxin WTX and its disulfide-fixed synthetic fragments: Interaction with nicotinic acetylcholine receptors and reduction of blood pressure

Non-conventional snake venom toxins, such as WTX from the cobra Naja kaouthia, are three-finger proteins containing a fifth disulfide bond in the N-terminal polypeptide loop I and inhibiting α7 and muscle-type nicotinic acetylcholine receptors (nAChRs). Because the central polypeptide loop II of non-conventional toxins plays an important role in their biological activity, we synthesized several WTX loop II fragments with two cysteine residues added at the N- and C-termini and oxidized to form a disulfide bond. The inhibition by peptides of several nAChRs subtypes was investigated using different methods and the effects of peptides on the rat arterial pressure and heart rate were analyzed. The synthetic fragments inhibited α7 and muscle-type nAChRs more potently than WTX. We showed for the first time that WTX and its fragments inhibited α9α10 as well as neuronal α3β2 and α4β2 nAChRs, again the synthetic fragments being more potent than WTX. The loop II fragments reduced blood pressure more potently than WTX in normotensive, awake rats. In connection with this, the WTX cardiovascular effects were analyzed and it was found that toxin very weakly affected parameters of papillary muscle contractions with no influence on aortic ring contractility. The observed effects were not so significant to explain the decrease in BP, the hemodynamic effects of WTX appearing not to result from direct influence on the myocardium and blood vessels. The synthetic fragments of the N- and C-terminal loops I and III were inactive in all tests. Thus, both in inhibition of all analyzed nAChR subtypes and in reduction of blood pressure, fragments of the central loop II were more active than WTX. This appears to be a first indication for three-finger proteins that the fragments of the central loop II are more active than the native toxin.

Combination with a Low Dose of Doxorubicin Further Boosts the Antitumor Effect of SLURP-1 In Vivo and Associates with EGFR Down-Regulation

Skin cancers such as squamous cell carcinoma (SCC) are among the most aggressive types of tumors. They come with a high rate of growth, metastasis, and frequently occurring chemoresistance. Smoking is one of the risk factors for SCC progression, and the α7 nicotinic acetylcholine receptor (α7-nAChR) is a promising target for SCC therapy. Human secreted protein SLURP-1 is an auto/paracrine regulator of epithelial homeostasis and a selective negative allosteric modulator of α7-nAChR. Recently, we demonstrated the high efficiency of the therapy based on the recombinant SLURP-1 in controlling SCC cell growth and metastasis in vivo. The anti-tumor effect of SLURP-1 was mediated through interaction with both α7-nAChR and the epidermal growth factor receptor (EGFR). Cytotoxic antibiotic doxorubicin has been proposed for the SCC therapy; however, its use is limited due to the high toxicity. In this study we investigated the use of an enhanced SLURP-1 dose and of a combination of SLURP-1 with low-dozen doxorubicin for SCC treatment of mice xenografted with squamous cell carcinoma A431 cells. An increased SLURP-1 dose didn't significantly enhance the efficiency of the therapy. However, the combination with doxorubicin further enhanced the anti- tumor activity of SLURP-1 and dramatically suppressed metastasis. The effect from the combined therapy was accompanied by down-regulation of EGFR expression in tumors. Direct inhibition of EGFR activation by SLURP-1 was shown. No toxicity of the combined therapy was encountered. Our data indicate that the combination of SLURP-1 with chemotherapy in lower doses is a promising approach in SCC treatment and should be further studied.

In Search of the Role of Three-Finger Starfish Proteins

Three-finger proteins (TFPs), or Ly6/uPAR proteins, are characterized by the beta-structural LU domain containing three protruding "fingers" and stabilized by four conserved disulfide bonds. TFPs were initially characterized as snake alpha-neurotoxins, but later many studies showed their regulatory roles in different organisms. Despite a known expression of TFPs in vertebrates, they are poorly studied in other taxa. The presence of TFPs in starfish was previously shown, but their targets and functional role still remain unknown. Here, we analyzed expression, target, and possible function of the Lystar5 protein from the Asterias rubens starfish using bioinformatics, qPCR, and immunoassay. First, the presence of Lystar5 homologues in all classes of echinoderms was demonstrated. qPCR revealed that mRNA of Lystar5 and LyAr2 are expressed mainly in coelomocytes and coelomic epithelium of Asterias, while mRNA of other TFPs, LyAr3, LyAr4, and LyAr5, were also found in a starfish body wall. Using anti-Lystar5 serum from mice immunized by a recombinant Lystar5, we confirmed that this protein is expressed on the surface of coelomocytes and coelomic epithelium cells. According to ELISA, a recombinant analogue of Lystar5 bound to the membrane fraction of coelomocytes and coelomic epithelium but not to the body wall or starfish arm tip. Analysis by LC-MALDI MS/MS suggested integrin α-8-like protein expressed in the coelomocytes and coelomic epithelium as a target of Lystar5. Thus, our insights propose the important role of TFPs in regulation of starfish physiology and show prospects for their further research.

Peptide Mimicking Loop II of the Human Epithelial Protein SLURP-2 Enhances the Viability and Migration of Skin Keratinocytes

The secreted human protein SLURP-2 is a regulator of epithelial homeostasis, which enhances the viability and migration of keratinocytes. The targets of SLURP-2 in keratinocytes are nicotinic and muscarinic acetylcholine receptors. This work is devoted to the search for the SLURP-2 functional regions responsible for enhancing keratinocyte viability and migration. We produced synthetic peptides corresponding to the SLURP-2 loop regions and studied their effect on the viability and migration of HaCaT skin keratinocytes using the WST-8 test and scratch-test, respectively. The highest activity was exhibited by a loop II-mimicking peptide that enhanced the viability of keratinocytes and stimulated their migration. The peptide activity was mediated by interactions with α7- and α3β2-nAChRs and suppression of the p38 MAPK intracellular signaling pathway. Thus, we obtained new data that explain the mechanisms underlying SLURP-2 regulatory activity and indicate the promise of further research into loop II-mimicking peptides as prototypes of wound healing drugs.

Auxiliary protein and chaperone regulation of neuronal nicotinic receptor subtype expression and function

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of pentameric, ligand-gated ion channels that are located on the surface of neurons and non-neuronal cells and have multiple physiological and pathophysiological functions. In order to reach the cell surface, many nAChR subtypes require the help of chaperone and/or auxiliary/accessory proteins for their assembly, trafficking, pharmacological modulation, and normal functioning in vivo. The use of powerful genome-wide cDNA screening has led to the identification and characterisation of the molecules and mechanisms that participate in the assembly and trafficking of receptor subtypes, including chaperone and auxiliary or accessory proteins. The aim of this review is to describe the latest findings concerning nAChR chaperones and auxiliary proteins and pharmacological chaperones, and how some of them control receptor biogenesis or regulate channel activation and pharmacology. Some auxiliary proteins are subtype selective, some regulate various subtypes, and some not only modulate nAChRs but also target other receptors and signalling pathways. We also discuss how changes in auxiliary proteins may be involved in nAChR dysfunctions.

Comparison of Conformations and Interactions with Nicotinic Acetylcholine Receptors for E. coli-Produced and Synthetic Three-Finger Protein SLURP-1

SLURP-1 is a three-finger human protein targeting nicotinic acetylcholine receptors (nAChRs). The recombinant forms of SLURP-1 produced in E. coli differ in added fusion fragments and in activity. The closest in sequence to the naturally occurring SLURP-1 is the recombinant rSLURP-1, differing by only one additional N-terminal Met residue. sSLURP-1 can be prepared by peptide synthesis and its amino acid sequence is identical to that of the natural protein. In view of recent NMR analysis of the conformational mobility of rSLURP-1 and cryo-electron microscopy structures of complexes of α-bungarotoxin (a three-finger snake venom protein) with Torpedo californica and α7 nAChRs, we compared conformations of sSLURP-1 and rSLURP-1 by Raman spectroscopy and CD-controlled thermal denaturation, analyzed their competition with α-bungarotoxin for binding to the above-mentioned nAChRs, compared the respective receptor complexes with computer modeling and compared their inhibitory potency on the α9α10 nAChR. The CD revealed a higher thermostability of sSLURP-1; some differences between sSLURP-1 and rSLURP-1 were observed in the regions of disulfides and tyrosine residues by Raman spectroscopy, but in binding, computer modeling and electrophysiology, the proteins were similar. Thus, sSLURP-1 and rSLURP-1 with only one additional Met residue appear close in structure and functional characteristics, being appropriate for research on nAChRs.