Interaction of calmodulin with synthetic deletion peptides of melittin

Melittin as a permeability enhancer II: in vitro investigations in human mucus secreting intestinal monolayers and rat colonic mucosae

PURPOSE

Melittin has shown potential as a non-cytotoxic absorption enhancer in Caco-2 monolayers. Our objectives were to assess in vitro efficacy and cytotoxicity of melittin in two intestinal permeability models and investigate the potential mechanism by which melittin might enhance gastrointestinal absorption.

MATERIALS AND METHODS

The effects of melittin were examined in the mucus-secreting intestinal cell monolayers, HT29-MTX-E12 (E12), using transepithelial electrical resistance (TER), transmission electron microscopy (TEM) and the MTT viability assay. The effects of melittin on TER, permeability and short circuit current (Isc) were also investigated in rat colon mucosae mounted in Ussing chambers. Ion transporting capacity of tissue was measured in response to secretagogues as surrogate markers of cytotoxicity. Melittin stability was examined by a means of a hemolytic assay. The mechanism by which melittin decreases TER across the rat mucosa was examined with a range of enzymatic inhibitors.

RESULTS

Apical addition of melittin resulted in a reversible non-cytotoxic concentration-dependent decrease in TER across E12 monolayers, which was independent of the presence of mucus. Apical addition of melittin reduced TER and increased the permeability of [(14)C]-mannitol across rat colonic mucosae. The melittin-induced drop in TER in rat colon was significantly attenuated by W7 suggesting partial mediation by calmodulin.

CONCLUSIONS

The rapid and reversible nature of melittin's permeation enhancing properties and its limited cytotoxicity in polarized intestinal epithelia, suggests a potential drug delivery role for the peptide in oral formulations of poorly absorbed drugs.

Deletion of two C-terminal Gln residues of 12-26-residue fragment of melittin improves its antimicrobial activity

In our previous paper it was shown that the two C-terminal Gln residues of a C-terminal 15-residue fragment, Mel(12-26) (GLPALISWIKRKRQQ-NH2), of melittin and a series of individual substituted analogues might not involved in the interaction with bacterial membranes. In this paper, peptides with one and two Gln residues deletion, respectively, Mel(12-25) and Mel(12-24), were synthesized and characterized. Both of the deletion peptides showed higher antimicrobial activities than the parent peptide, Mel(12-26). If both of the Gln residues of Mel(12-26) were respectively replaced by a hydrophilic amino acid Gly, the antimicrobial activity increased slightly. If the Gln residue of Mel(12-25) was replaced by a hydrophobic amino acid Leu, the antimicrobial activity changed little, although the substituted peptide possessed much higher hydrophobicity and higher alpha-helical conformation percentage in 1,1,1,3,3,3-hexafluoro-2-propanol/water determined by circular dichroism spectroscopy (CD) than the parent peptide. These results indicated that the two C-terminal residues might be indeed not involved in the binding to bacterial membranes. The antimicrobial activity increasing with the residue deletion may be caused by the decrease of the translational and rotational entropic cost of the binding of the peptides to bacterial membranes because of the lower molecular weights of the deletion peptides.

Individual substitution analogs of Mel(12-26), melittin's C-terminal 15-residue peptide: their antimicrobial and hemolytic actions

Residues 1-9 of M(12-26) (GLPALISWIKRKRQQ-NH2), the C-terminal 15-residue segment of melittin, were substituted individually to change the hydropathicities in these positions. Antimicrobial and hemolytic activities of these peptides were determined. The results showed increased antimicrobial activities with increased hydrophobicities at almost all the positions studied. The effects at positions 2, 5, 8 and 9 were significant while the effects at the other positions were small. These two groups of residues were located on the opposite faces of the alpha-helix. In other words, the hydrophobicities of the two faces were favorable, but one face (the more favorable face) contributed more to the antimicrobial activities than the other (the less favorable face). The hydrophobicity, not the amphipathicity, seems to be crucial for antimicrobial activity. In contrast, the hydrophobicity of one face was favorable but the other was unfavorable for the hemolytic activity, indicating that the amphipathicity may be important for hemolysis. Interestingly, the more favorable face for antimicrobial activity was located opposite to the favorable face for hemolytic activity, indicating the direction of the hydrophobic face for the antimicrobial activity and direction of the amphipathicity for the hemolytic activity were also important.

Effect of lipid-derived second messengers on electrophysiological taste responses in the gerbil

Integrated chorda tympani (CT) recordings were made to salty, sour, sweet, bitter, and glutamate tastants before and after a 4-min application of modulators of lipid-derived second messenger systems. The modulators included two membrane-permeable analogues of DAG, 1-oleoyl-2-acetyl glycerol (OAG) and dioctanoyl glycerol (DiC8); thapsigargin, which releases Ca++ from intracellular stores; ionomycin, a calcium ionophore; lanthanum chloride, an inorganic calcium channel blocker; nifedipine, a dihydropyridine calcium channel blocker; quinacrine diHCl, a phospholipase A2 antagonist; melittin, a phospholipase A2 agonist; and indomethacin, which decreases the release of prostaglandins by inhibiting the enzyme cyclo-oxygenase. The main findings were: OAG (125 microM) and DiC8 (100 microM) blocked the responses of several bitter compounds while enhancing the taste response to several sweeteners. Lanthanum chloride blocked all responses, which may be due to the fact that it blocks tight junctions. Quinacrine (1 mM) suppressed several bitter responses while enhancing the response to several sweeteners. The enhancement of sweet taste responses by DAG analogues suggests that there is cross-talk between the adenylate cyclase system and one (or more) pathways involving lipid-derived second messengers in taste cells.