Novel mastoparan analogs induce differential secretion from mast cells

The cationic tetradecapeptide mastoparan as a privileged structure for drug discovery: Enhanced antimicrobial properties of mitoparan analogues modified at position-14

Mastoparan (MP) peptides, distributed in insect venoms, induce a local inflammatory response post envenomation. Most endogenous MPs share common structural elements within a tetradecapeptide sequence that adopts an amphipathic helix whilst traversing biological membranes and when bound to an intracellular protein target. Rational modifications to increase cationic charge density and amphipathic helicity engineered mitoparan (MitP), a mitochondriotoxic bioportide and potent secretagogue. Following intracellular translocation, MitP is accreted by mitochondria thus indicating additional utility as an antimicrobial agent. Hence, the objectives of this study were to compare the antimicrobial activities of a structurally diverse set of cationic cell penetrating peptides, including both MP and MitP sequences, and to chemically engineer analogues of MitP for potential therapeutic applications. Herein, we confirm that, like MP, MitP is a privileged structure for the development of antimicrobial peptides active against both prokaryotic and eukaryotic pathogens. Collectively, MitP and target-selective chimeric analogues are broad spectrum antibiotics, with the Gram-negative A. baumannii demonstrating particular susceptibility. Modifications of MitP by amino acid substitution at position-14 produced peptides, Δ14MitP analogues, with unique pharmacodynamic properties. One example, [Ser¹⁴]MitP, lacks both cytotoxicity against human cell lines and mast cell secretory activity yet retains selective activity against the encapsulated yeast C. neoformans.

Cell penetrating peptide-mediated transport enables the regulated secretion of accumulated cargoes from mast cells

The in vivo utility of technologies employing cell penetrating peptides and bioportides may be compromised by the general capacity of polycationic peptides to activate mast cell secretion. Moreover, the same technologies could be exploited in a clinical setting either to directly modulate intrinsic exocytotic mechanisms or to load mast cells with bioactive cargoes. Comparative investigations identified two cell penetrating vectors, Tat and C105Y, which readily translocate into mast cells without inducing receptor-independent exocytosis. Efficient Tat transduction also enabled the intracellular delivery and accumulation of cargoes within discrete intracellular compartments. A tetramethylrhodamine-Tat conjugate is effectively translocated into the secretory lysosomes of RBL-2H3 cells. In contract, the intracellular delivery of avidin, as a non-covalent complex with a biotinylated Tat vector, is also efficient but the protein is predominantly accumulated outside of secretory lysosomes. Significantly, both cargoes can be subsequently released following mast cell stimulation either by mastoparan, a wasp venom secretagogue, or by the physiological mechanism of antigen-induced aggregation of high affinity IgE receptors. These studies indicate that mast cells could be exploited to direct the delivery of bioactive agents to disease sites as an innovative cell-mediated therapy.

Intracellular translocation of the decapeptide carboxyl terminal of Gi3 alpha induces the dual phosphorylation of p42/p44 MAP kinases

The carboxyl terminal of heterotrimeric G protein alpha subunits binds both G protein-coupled receptors and mastoparan (MP), a tetradecapeptide allostere. Moreover, peptides corresponding to the carboxyl domains of G(i)3alpha and G(t) display intrinsic biological activities in cell-free systems. Thus, the purpose of this study was to develop a cell penetrant delivery system to further investigate the biological properties of a peptide mimetic of the G(i)3alpha carboxyl terminal (G(i)3alpha(346-355); H-KNNLKECGLY-NH2). Kinetic studies, using a CFDA-conjugated analogue of G(i)3alpha(346-355), confirmed the rapid and efficient intracellular translocation of TP10-G(i)3alpha(346-355) (t(0.5) = 3 min). Translocated G(i)3alpha(346-355), but not other bioactive cargoes derived from PKC and the CB1 cannabinoid receptor, promoted the dual phosphorylation of p42/p44 MAPK without adverse changes in cellular viability. The relative specificity of this novel biological activity was further confirmed by the observation that translocated G(i)3alpha(346-355) did not influence the exocytosis of beta-hexoseaminidase from RBL-2H3, a secretory event stimulated by other cell penetrant peptide cargoes and MP. We conclude that TP10-G(i)3alpha(346-355) is a valuable, non-toxic research tool with which to study and modulate signal transduction pathways mediated by heterotrimeric G proteins and MAPK.

Charge delocalisation and the design of novel mastoparan analogues: enhanced cytotoxicity and secretory efficacy of [Lys5, Lys8, Aib10]MP

The formation of an amphipathic helix is a major determinant of the biological activity of the tetradecapeptide mastoparan (MP). To address the functional significance of lysyl residues at positions 4, 11 and 12 of MP, we synthesised five novel analogues using sequence permutation and arginine-substitution to delocalise cationic charge. Comparative bioassays determined cytotoxicity, beta-hexoseaminidase secretory efficacy and peptide-activated extracellular receptor-stimulated kinase (ERK)1/2 phosphorylation. The monosubstitution of individual lysine residues with arginine produced differential changes to the indices of cytotoxicity and secretion indicating that these conservative substitutions are compatible with membrane translocation and the selective binding and activation of intracellular proteins. More profound changes to the predicted hydrophilic face of MP, resulting from the relocation or substitution of additional lysyl residues, enhanced both the cytotoxicity and secretory efficacy of novel peptides. Significantly, the more amphipathic peptide [Lys5, Lys8, Aib10]MP was identified to be both the most cytotoxic and the most potent secretagogue of all the peptides compared here. Charge delocalisation within the hydrophilic face of MP analogues was also compatible with peptide-induced activation of ERK1/2 phosphorylation. Our data indicate that charge delocalisation is a suitable strategy to engineer more potent analogues of MP that differentially target intracellular proteins.

Intracellular Delivery of Bioactive Peptides to RBL-2H3 Cells Induces β-Hexosaminidase Secretion and Phospholipase D Activation

This investigation compared the secretory efficacies of a series of peptides delivered to the cytoplasm of RBL-2H3 mast cells. Mimetic peptides, designed to target intracellular proteins that regulate cell signalling and membrane fusion, were synthesised as transportan 10 (TP10) chimeras for efficient plasma membrane translocation. Exocytosis of beta-hexosaminidase, a secretory lysosomal marker, indicated that peptides presenting sequences derived from protein kinase C (PKC; C1 H-CRRLSVEIWDWDL-NH(2)) and the CB(1) cannabinoid receptor (C3 H-RSKDLRHAFRSMFPSCE-NH(2)) induced beta-hexosaminidase secretion. Other peptide cargoes, including a Rab3A-derived sequence and a homologue of C3, were inactive in similar assays. Translocated C1 also activated phospholipase D (PLD), an enzyme intimately involved in the regulated secretory response of RBL-2H3 cells, but C1-induced secretion was not dependent upon phosphatidate synthesis. Neither down-regulation of Ca(2+)-sensitive isoforms of PKC nor the application of a selective PKC inhibitor attenuated the secretory efficacy of C1. These observations indicate that the molecular target of C1 is a protein involved in the regulated secretory pathway that is upstream of PLD but is not a PKC isoform. This study also confirmed that TP10 is a relatively inert cell-penetrating vector and is, therefore, widely suitable for studies in cells that are sensitive to peptidyl secretagogues.

Mastoparan selectively activates phospholipase D2 in cell membranes

Both known isoforms of phospholipase (PL) D, PLD1 and PLD2, require phosphatidylinositol 4,5-bisphosphate for activity. However, PLD2 is fully active in the presence of this phospholipid, whereas PLD1 activation is dependent on additional factors such as ADP-ribosylation factor-1 (ARF-1) and protein kinase Calpha. We find that mastoparan, an activator of G(i) and mast cells, stimulates an intrinsic PLD activity, most likely PLD2, in fractions enriched in plasma membranes from rat basophilic leukemia 2H3 mast cells. Overexpression of PLD2, but not of PLD1, results in a large increase in the mastoparan-inducible PLD activity in membrane fractions, particularly those enriched in plasma membranes. As in previous studies, expressed PLD2 is localized primarily in the plasma membrane and PLD1 in granule membranes. Studies with pertussis toxin and other agents indicate that mastoparan stimulates PLD2 independently of G(i), ARF-1, protein kinase C, and calcium. Kinetic studies indicate that mastoparan interacts synergistically with phosphatidylinositol 4,5-bisphosphate and that oleate, itself a weak stimulant of PLD2 at low concentrations, is a competitive inhibitor of mastoparan stimulation of PLD2. Therefore, mastoparan may be useful for investigating the regulation of PLD2, particularly in view of the well studied molecular interactions of mastoparan with certain other strategic signaling proteins.