Membrane leakage induced by dynorphins

Probing electrophysiological activity of amphiphilic Dynorphin A in planar neutral membranes reveals both ion channel-like activity and neuropeptide translocation

Dynorphin A (DynA) is an endogenous neuropeptide that besides acting as a ligand of the κ-opioid receptor, presents some non-opioid pathophysiological properties associated to its ability to induce cell permeability similarly to cell-penetrating peptides (CPPs). Here, we use electrophysiology experiments to show that amphiphilic DynA generates aqueous pores in neutral membranes similar to those reported previously in charged membranes, but we also find other events thermodynamically incompatible with voltage-driven ion channel activity (i.e. non-zero currents with no applied voltage in symmetric salt conditions, reversal potentials that exceed the theoretical limit for a given salt concentration gradient). By comparison with current traces generated by other amphiphilic molecule known to spontaneously cross membranes, we hypothesize that DynA could directly translocate across neutral bilayers, a feature never observed in charged membranes following the same electrophysiological protocol. Our findings suggest that DynA interaction with the cellular membrane is modulated by the lipid charge distribution, enabling either passive ionic transport via membrane remodeling and pore formation or by peptide direct internalization independent of cellular transduction pathways.

Dynorphin A induces membrane permeabilization by formation of proteolipidic pores. Insights from electrophysiology and computational simulations

Dynorphins are endogenous neuropeptides that function as ligands for the κ-opioid receptor. In addition to opioid activity, dynorphins can induce several pathological effects such as neurological dysfunctions and cell death. Previous studies have suggested that Dynorphin A (DynA) mediates some pathogenic actions through formation of transient pores in lipid domains of the plasma membrane. Here, we use planar bilayer electrophysiology to show that DynA induces pore formation in negatively charged membranes. We find a large variability in pore conformations showing equilibrium conductance fluctuations, what disregards electroporation as the dominant mechanism of pore formation. Ion selectivity measurements showing cationic selectivity indicate that positive protein charges of DynA are stabilized by phosphatidyl serine negative charges in the formation of combined structures. We complement our study with computational simulations that assess the stability of diverse peptide arrangements in the hydrophobic core of the bilayer. We show that DynA is capable of assembling in charged membranes to form water-filled pores that conduct ions.

Effects of Antibacterial Peptide F1 on Bacterial Liposome Membrane Integrity

Previous studies from our lab have shown that the antimicrobial peptide F1 obtained from the milk fermentation by Lactobacillus paracasei FX-6 derived from Tibetan kefir was different from common antimicrobial peptides; specifically, F1 simultaneously inhibited the growth of Gram-negative and Gram-positive bacteria. Here, we present follow-on work demonstrating that after the antimicrobial peptide F1 acts on either Escherichia coli ATCC 25922 (E. coli) or Staphylococcus aureus ATCC 63589 (S. aureus), their respective bacterial membranes were severely deformed. This deformation allowed leakage of potassium and magnesium ions from the bacterial membrane. The interaction between the antimicrobial peptide F1 and the bacterial membrane was further explored by artificially simulating the bacterial phospholipid membranes and then extracting them. The study results indicated that after the antimicrobial peptide F1 interacted with the bacterial membranes caused significant calcein leakage that had been simulated by different liposomes. Furthermore, transmission electron microscopy observations revealed that the phospholipid membrane structure was destroyed and the liposomes presented aggregation and precipitation. Quartz Crystal Microbalance with Dissipation (QCM-D) results showed that the antimicrobial peptide F1 significantly reduced the quality of liposome membrane and increased their viscoelasticity. Based on the study's findings, the phospholipid membrane particle size was significantly increased, indicating that the antimicrobial peptide F1 had a direct effect on the phospholipid membrane. Conclusively, the antimicrobial peptide F1 destroyed the membrane structure of both Gram-negative and Gram-positive bacteria by destroying the shared components of their respective phospholipid membranes which resulted in leakage of cell contents and subsequently cell death.

Antibacterial action of peptide F1 against colistin resistance E. coli SHP45 (mcr-1)

The emergence of the plasmid-mediated colistin resistance mechanism (mcr-1) makes bacterial resistance to colistin increasingly serious. This mcr-1 mediated bacterial resistance to colicin is conferred primarily through modification of lipid A in lipopolysaccharides (LPS). In our previous research, antimicrobial peptide F1 was derived from Tibetan kefir and has been shown to effectively inhibit the growth of Gram-negative bacteria (E. coli), Gram-positive bacteria (Staphylococcus aureus), and other pathogenic bacteria. Based on this characteristic of antibacterial peptide F1, we speculated that it could inhibit the growth of the colicin-resistant E. coli SHP45 (mcr-1) and not easily produce drug resistance. Studies have shown that antimicrobial peptide F1 can destroy the liposome structure of the phospholipid bilayer by destroying the inner and outer membranes of bacteria, thereby significantly inhibiting the growth of E. coli SHP45 (mcr-1), but without depending on LPS. The results of this study confirmed our hypothesis, and we anticipate that antimicrobial peptide F1 will become a safe antibacterial agent that can assist in solving the problem of drug resistance caused by colistin.

Spinocerebellar ataxia type 23 (SCA23): a review

Spinocerebellar ataxias (SCAs), formerly known as autosomal dominant cerebellar ataxias (ADCAs), are a group of hereditary heterogeneous neurodegenerative diseases. Gait, progressive ataxia, dysarthria, and eye movement disorder are common symptoms of spinocerebellar ataxias. Other symptoms include peripheral neuropathy, cognitive impairment, psychosis, and seizures. Patients may lose their lives due to out of coordinated respiration and/or swallowing. Neurological signs cover pyramidal or extrapyramidal signs, spasm, ophthalmoplegia, hyperactive deep tendon reflexes, and so on. Different subtypes of SCAs present various clinical features. Spinocerebellar ataxia type 23 (SCA23), one subtype of the SCA family, is characterized by mutant prodynorphin (PDYN) gene. Based on literatures, this review details a series of SCA23, to improve a whole understanding of clinicians and point out the potential research direction of this dysfunction, including a history, pathophysiological mechanism, diagnosis and differential diagnosis, epigenetics, penetrance and prevalence, genetic counseling, treatment and prognosis.

Alanine scan of the opioid peptide dynorphin B amide

To date structure-activity relationship (SAR) studies of the dynorphins (Dyn), endogenous peptides for kappa opioid receptors (KOR), have focused almost exclusively on Dyn A with minimal studies on Dyn B. While both Dyn A and Dyn B have identical N-terminal sequences, their C-terminal sequences differ, which could result in differences in pharmacological activity. We performed an alanine scan of the non-glycine residues up through residue 11 of Dyn B amide to explore the roles of these side chains in the activity of Dyn B. The analogs were synthesized by fluorenylmethyloxycarbonyl (Fmoc)-based solid phase peptide synthesis and evaluated for their opioid receptor affinities and opioid potency and efficacy at KOR. Similar to Dyn A the N-terminal Tyr¹ and Phe⁴ residues of Dyn B amide are critical for opioid receptor affinity and KOR agonist potency. The basic residues Arg⁶ and Arg⁷ contribute to the KOR affinity and agonist potency of Dyn B amide, while Lys¹⁰ contributes to the opioid receptor affinity, but not KOR agonist potency, of this peptide. Comparison to the Ala analogs of Dyn A (1-13) suggests that the basic residues in the C-terminus of both peptides contribute to KOR binding, but differences in their relative positions may contribute to the different pharmacological profiles of Dyn A and Dyn B. The other unique C-terminal residues in Dyn B amide also appear to influence the relative affinity of this peptide for KOR versus mu and delta opioid receptors. This SAR information may be applied in the design of new Dyn B analogs that could be useful pharmacological tools.

An acidic model pro-peptide affects the secondary structure, membrane interactions and antimicrobial activity of a crotalicidin fragment

In order to study how acidic pro-peptides inhibit the antimicrobial activity of antimicrobial peptides, we introduce a simple model system, consisting of a 19 amino-acid long antimicrobial peptide, and an N-terminally attached, 10 amino-acid long acidic model pro-peptide. The antimicrobial peptide is a fragment of the crotalicidin peptide, a member of the cathelidin family, from rattlesnake venom. The model pro-peptide is a deca (glutamic acid). Attachment of the model pro-peptide only leads to a moderately large reduction in the binding to- and induced leakage of model liposomes, while the antimicrobial activity of the crotalicidin fragment is completely inhibited by attaching the model pro-peptide. Attaching the pro-peptide induces a conformational change to a more helical conformation, while there are no signs of intra- or intermolecular peptide complexation. We conclude that inhibition of antimicrobial activity by the model pro-peptide might be related to a conformational change induced by the pro-peptide domain, and that additional effects beyond induced changes in membrane activity must also be involved.

Plasma membrane poration by opioid neuropeptides: a possible mechanism of pathological signal transduction

Neuropeptides induce signal transduction across the plasma membrane by acting through cell-surface receptors. The dynorphins, endogenous ligands for opioid receptors, are an exception; they also produce non-receptor-mediated effects causing pain and neurodegeneration. To understand non-receptor mechanism(s), we examined interactions of dynorphins with plasma membrane. Using fluorescence correlation spectroscopy and patch-clamp electrophysiology, we demonstrate that dynorphins accumulate in the membrane and induce a continuum of transient increases in ionic conductance. This phenomenon is consistent with stochastic formation of giant (~2.7 nm estimated diameter) unstructured non-ion-selective membrane pores. The potency of dynorphins to porate the plasma membrane correlates with their pathogenic effects in cellular and animal models. Membrane poration by dynorphins may represent a mechanism of pathological signal transduction. Persistent neuronal excitation by this mechanism may lead to profound neuropathological alterations, including neurodegeneration and cell death.

Selectivity for and destruction of Salmonella typhimurium via a membrane damage mechanism of a cell-penetrating peptide ppTG20 analogue

P7, an analogue of the cell-penetrating peptide (CPP) ppTG20, was derived by replacing Phe and Trp with Arg based on the structure-activity relationships of CPPs and antimicrobial peptides (AMPs). P7 showed antimicrobial activity against Salmonella typhimurium at 4 μM and possessed broad antimicrobial activity greater than its parent peptide. P7 displayed good selectivity, with low haemolysis below its minimum inhibitory concentration range, but displayed cytotoxic activity against the HT29 and MDA-MB231 mammalian cell lines. Studies of calcein leakage from egg yolk L-α-phosphatidylcholine/egg yolk L-α-phosphatidyl-DL-glycerol (EYPC/EYPG) (bacterial membrane mimic) and EYPC/cholesterol (eukaryotic membrane mimic) vesicles also demonstrated that P7 exhibited high selectivity and caused pore formation in the bacterial membrane. Circular dichroism experiments suggested that the conformation of P7 underwent transitions from a random coil in sodium phosphate buffer to an α-helical conformation in bacterial model membranes. P7 induced influx of the membrane fluorescent probe 1-N-phenylnaphthylamine (NPN) and the nucleic acid fluorescent probe SYTOX(®) Green by increasing live S. typhimurium cell outer membrane and plasma membrane permeability, respectively. P7 also induced ion channel formation in the cell plasma membrane causing leakage of potassium ions. Flow cytometric analysis demonstrated that S. typhimurium cell membrane integrity was destroyed following incubation with P7. These results indicated that P7 exhibited good bacterial selectivity and exerted its antibacterial activity by a membrane damage mechanism. Furthermore, these results suggested that CPPs may represent a source of templates for AMP design.

Non-opioid nociceptive activity of human dynorphin mutants that cause neurodegenerative disorder spinocerebellar ataxia type 23

We previously identified four missense mutations in the prodynorphin gene that cause human neurodegenerative disorder spinocerebellar ataxia type 23 (SCA23). Three mutations substitute Leu(5), Arg(6), and Arg(9) to Ser (L5S), Trp (R6W) and Cys (R9C) in dynorphin A(1-17) (Dyn A), a peptide with both opioid activities and non-opioid neurodegenerative actions. It has been reported that Dyn A administered intrathecally (i.t.) in femtomolar doses into mice produces nociceptive behaviors consisting of hindlimb scratching along with biting and licking of the hindpaw and tail (SBL responses) through a non-opioid mechanism. We here evaluated the potential of the three mutant peptides to produce similar behaviors. Compared to the wild type (WT)-peptide, the relative potency of Dyn A R6W, L5S and R9C peptides for SBL responses was 50-, 33- and 2-fold higher, and Dyn A R6W and L5S induced the SBL responses at a 10-30-fold lower doses. Dyn A R6W was the most potent peptide. The SBL responses induced by Dyn A R6W were dose dependently inhibited by morphine (i.p.; 0.1-1 mg/kg) or MK-801, an NMDA ion channel blocker (i.t. co-administration; 5-7.5 nmol). CP-99,994, a tachykinin NK1 receptor antagonist (i.t. co-administration; 2 nmol) and naloxone (i.p.; 5 mg/kg) failed to block effects of Dyn A R6W. Thus, similarly to Dyn A WT, the SBL responses induced by Dyn A R6W may involve the NMDA receptor but are not mediated through the opioid and tachykinin NK1 receptors. Enhanced non-opioid excitatory activities of Dyn A mutants may underlie in part development of SCA23.

Substrate specificity within a family of outer membrane carboxylate channels

Characterization of a large family of outer membrane channels from gram-negative bacteria suggest how they can thrive in nutrient-poor environments and how channel inactivation can contribute to antibiotic resistance.

Many Gram-negative bacteria, including human pathogens such as Pseudomonas aeruginosa, do not have large-channel porins. This results in an outer membrane (OM) that is highly impermeable to small polar molecules, making the bacteria intrinsically resistant towards many antibiotics. In such microorganisms, the majority of small molecules are taken up by members of the OprD outer membrane protein family. Here we show that OprD channels require a carboxyl group in the substrate for efficient transport, and based on this we have renamed the family Occ, for outer membrane carboxylate channels. We further show that Occ channels can be divided into two subfamilies, based on their very different substrate specificities. Our results rationalize how certain bacteria can efficiently take up a variety of substrates under nutrient-poor conditions without compromising membrane permeability. In addition, they explain how channel inactivation in response to antibiotics can cause resistance but does not lead to decreased fitness.

Emerging antibiotic resistance in the treatment of infectious disease is an increasing problem that urgently requires new drug development. In Gram-negative bacteria, the outer membrane (OM) prevents permeation of small molecules, including antibiotics. A family of channel-forming proteins, called OprD proteins, are present in the OM to enable uptake of nutrients required for growth and cellular function. Since these channels also transport antibiotics, understanding how molecules are recognized and transported by these proteins should enable the design of more effective antibiotics. Here, we have characterized by biophysical and biochemical methods the structures and substrate-specificities of nine members of the OprD channel family of a common multidrug-resistant pathogen, Pseudomonas aeruginosa. Because we demonstrate that efficient passage through these channels requires the presence of a carboxyl group in the substrate, we renamed this channel family outer membrane carboxylate channels, or Occ. This broad substrate specificity suggests that such efficient transport allows bacteria to thrive in nutrient-poor environments. We also show markedly varied substrate specificities among the family members, especially for antibiotics, suggesting that mutation of a single channel can result in antibiotic resistance. These results provide the framework for studying the interaction of antibiotics with OM uptake channels, which will facilitate the development of more permeable and thus effective drugs.

The effect of the placement and total charge of the basic amino acid clusters on antibacterial organism selectivity and potency

Extensive circular dichroism, isothermal titration calorimetry and induced calcein leakage studies were conducted on a series of antimicrobial peptides (AMPs), with a varying number of Lys residues located at either the C-terminus or the N-terminus to gain insight into their effect on the mechanisms of binding with zwitterionic and anionic membrane model systems. Different CD spectra were observed for these AMPs in the presence of zwitterionic DPC and anionic SDS micelles indicating that they adopt different conformations on binding to the surfaces of zwitterionic and anionic membrane models. Different CD spectra were observed for these AMPs in the presence of zwitterionic POPC and anionic mixed 4:1 POPC/POPG LUVs and SUVs, indicating that they adopt very different conformations on interaction with these two types of LUVs and SUVs. In addition, ITC and calcein leakage data indicated that all the AMPs studied interact via very different mechanisms with anionic and zwitterionic LUVs. ITC data suggest these peptides interact primarily with the surface of zwitterionic LUVs while they insert into and form pores in anionic LUVs. CD studies indicated that these compounds adopt different conformations depending on the ratio of POPC to POPG lipids present in the liposome. There are detectable spectroscopic and thermodynamic differences between how each of these AMPs interacts with membranes, that is position and total charge density defines how these AMPs interact with specific membrane models and thus partially explain the resulting diversity of antibacterial activity of these compounds.

Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23

Spinocerebellar ataxias (SCAs) are dominantly inherited neurodegenerative disorders characterized by progressive cerebellar ataxia and dysarthria. We have identified missense mutations in prodynorphin (PDYN) that cause SCA23 in four Dutch families displaying progressive gait and limb ataxia. PDYN is the precursor protein for the opioid neuropeptides, α-neoendorphin, and dynorphins A and B (Dyn A and B). Dynorphins regulate pain processing and modulate the rewarding effects of addictive substances. Three mutations were located in Dyn A, a peptide with both opioid activities and nonopioid neurodegenerative actions. Two of these mutations resulted in excessive generation of Dyn A in a cellular model system. In addition, two of the mutant Dyn A peptides induced toxicity above that of wild-type Dyn A in cultured striatal neurons. The fourth mutation was located in the nonopioid PDYN domain and was associated with altered expression of components of the opioid and glutamate system, as evident from analysis of SCA23 autopsy tissue. Thus, alterations in Dyn A activities and/or impairment of secretory pathways by mutant PDYN may lead to glutamate neurotoxicity, which underlies Purkinje cell degeneration and ataxia. PDYN mutations are identified in a small subset of ataxia families, indicating that SCA23 is an infrequent SCA type (∼0.5%) in the Netherlands and suggesting further genetic SCA heterogeneity.

Spectroscopic and thermodynamic evidence for antimicrobial peptide membrane selectivity

In our laboratory we developed a series of antimicrobial peptides that exhibit selectivity and potency for prokaryotic over eukaryotic cells (Hicks et al., 2007). Circular dichroism (CD), isothermal calorimetry (ITC) and calcein leakage assays were conducted to determine the mechanism of lipid binding of a representative peptide 1 (Ac-GF-Tic-Oic-GK-Tic-Oic-GF-Tic-Oic-GK-Tic-KKKK-CONH(2)) to model membranes. POPC liposomes were used as a simple model for eukaryotic membranes and 4:1 POPC:POPG liposomes were used as a simple model for prokaryotic membranes. CD, ITC and calcein leakage data clearly indicate that compound 1 interacts via very different mechanisms with the two different liposome membranes. Compound 1 exhibits weaker binding and induces less calcein leakage in POPC liposomes than POPC:POPG (4:1 mole ratio) liposomes. The predominant binding mechanism to POPC appears to be limited to surface interactions while the mechanism of binding to 4:1 POPC:POPG most likely involves some type of pore formation.

Calcium influx into phospholipid vesicles caused by dynorphin neuropeptides

Dynorphins, endogeneous opioid peptides, function as ligands to the opioid kappa receptors but also induce non-opioid excitotoxic effects. Dynorphin A can increase the intra-neuronal calcium concentration through a non-opioid and non-NMDA mechanism. In this investigation, we show that big dynorphin, dynorphin A and to some extent dynorphin A (1-13), but not dynorphin B, allow calcium to enter into large unilamellar phospholipid vesicles with partly negative headgroups. The effects parallel the previously studied potency of dynorphins to translocate through biological membranes and to cause calcein leakage from large unilamellar phospholipid vesicles. There is no calcium ion influx into vesicles with zwitterionic headgroups. We have also investigated if the dynorphins can translocate through the vesicle membranes and estimated the relative strength of interaction of the peptides with the vesicles by fluorescence resonance energy transfer. The results show that dynorphins do not translocate in this membrane model system. There is a strong electrostatic contribution to the interaction of the peptides with the membrane model system.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Dysregulation of dynorphins in Alzheimer disease

The opioid peptides dynorphins may be involved in pathogenesis of Alzheimer disease (AD) by inducing neurodegeneration or cognitive impairment. To test this hypothesis, the dynorphin system was analyzed in postmortem samples from AD and control subjects, and subjects with Parkinson or cerebro-vascular diseases for comparison. Dynorphin A, dynorphin B and related neuropeptide nociceptin were determined in the Brodmann area 7 by radioimmunoassay. The precursor protein prodynorphin, processing convertase PC2 and the neuroendocrine pro7B2 and 7B2 proteins required for PC2 maturation were analyzed by Western blot. AD subjects displayed robustly elevated levels of dynorphin A and no differences in dynorphin B and nociceptin compared to controls. Subjects with Parkinson or cerebro-vascular diseases did not differ from controls with respect to any of the three peptides. PC2 levels were also increased, whereas, those of prodynorphin and pro7B2/7B2 were not changed in AD. Dynorphin A levels correlated with the neuritic plaque density. These results along with the known non-opioid ability of dynorphin A to induce neurodegeneration suggest a role for this neuropeptide in AD neuropathology.

Membrane Interactions of Dynorphins,

The dynorphins are primarily endogenous ligands to the kappa-opioid receptor, but a variety of non-opioid effects have also been observed, including direct effects on membranes. The peptides are rich in Arg residues, a characteristic feature of the cell-penetrating peptides. In this investigation, we have examined the interaction of the two peptides dynorphin A and dynorphin B with model membranes. A variety of NMR methods, as well as CD and fluorescence spectroscopy, have been used to characterize the structure of the two peptides and, more importantly, the position of the peptides in phospholipid bicelles. Both peptides interact to a large extent with both zwitterionic and partly negatively charged bicelles but are only marginally structured in either solvent. Dynorphin A was found to insert its N-terminus into the bilayer of the bicelle, while dynorphin B was found to reside on the surface of the bilayer. Despite the high degree of similarity in the sequence of the two peptides, it has previously been observed that dynorphin A has membrane perturbing effects and causes leakage of calcein from large unilamellar phospholipid vesicles while dynorphin B has no such effects. Our results provide a possible explanation for the difference in membrane perturbation.