Current Drug Targets - Volume 13, Issue 9, 2012

The first barrier that an antimicrobial agent must overcome when interacting with its target is the microbial cell wall. In the case of Gram-negative bacteria, additional to the cytoplasmic membrane and the peptidoglycan layer, an outer membrane (OM) is the outermost barrier. The OM has an asymmetric distribution of the lipids with phospholipids and lipopolysaccharide (LPS) located in the inner and outer leaflets, respectively. In contrast, Gram-positive bacteria lack OM and possess a much thicker peptidoglycan layer compared to their Gram-negative counterparts. An additional class of amphiphiles exists in Gram-positives, the lipoteichoic acids (LTA), which may represent important structural components. These long molecules cross-bridge the entire cell envelope with their lipid component inserting into the outer leaflet of the cytoplasmic membrane and the teichoic acid portion penetrating into the peptidoglycan layer. Furthermore, both classes of bacteria have other important amphiphiles, such as lipoproteins, whose importance has become evident only recently. It is not known yet whether any of these amphiphilic components are able to stimulate the immune system under physiological conditions as constituents of intact bacteria. However, all of them have a very high pro-inflammatory activity when released from the cell. Such a release may take place through the interaction with the immune system, or with antibiotics (particularly with those targeting cell wall components), or simply by the bacterial division. Therefore, a given antimicrobial agent must ideally have a double character, namely, it must overcome the bacterial cell wall barrier, without inducing the liberation of the pro-inflammatory amphiphiles. Here, new data are presented which describe the development and use of membrane-active antimicrobial agents, in particular antimicrobial peptides (AMPs) and lipopolyamines. In this way, essential progress was achieved, in particular with respect to the inhibition of deleterious consequences of bacterial infections such as severe sepsis and septic shock.

The bacterial cell wall represents the primary target for antimicrobial agents. Microbial destruction is accompanied by the release of potent immunostimulatory membrane constituents. Both Gram-positive and Gram-negative bacteria release a variety of lipoproteins and peptidoglycan fragments. Gram-positive bacteria additionally provide lipoteichoic acids, whereas Gram-negative bacteria also release lipopolysaccharide (LPS, endotoxin), essential component of the outer leaflet of the bacterial cell wall and one of the most potent immunostimulatory molecules known. Immune activation therefore can be considered as an adverse effect of antimicrobial destruction and killing during anti-infective treatment. In contrast to antibiotics, the use of cationic amphiphilic antimicrobial peptides allows both effective bacterial killing and inhibition of the immunostimulatory effect of the released bacterial membrane constituents. The administration of antimicrobial peptides alone or in combination with antibiotic agents thus represents a novel strategy in the antiinfective treatment with potentially important beneficial aspects. Here, data are presented which describe immunological and clinical aspects of the use of antimicrobial peptides (AMPs) as therapeutic agents to treat bacterial infection and neutralize the immunostimulatory activity of released cell wall constituents.

Diseases caused by protozoan parasites can pose a severe thread to human health and are behind some serious neglected tropical diseases like malaria and leishmaniasis. Though several different drugs have been developed in order to eradicate these diseases, a successful candidate has not yet been discovered. Among the most active compounds tested, antimicrobial peptides (AMPs) are particularly appealing because of their wide spectrum of action. AMPs have been described to perturb protozoan homeostasis by disrupting the cellular membranes but also by interfering with key processes in the parasite metabolism. In this review we describe the diverse mechanisms of action of AMPs on protozoan targets and how they can be exploited to treat diseases. Moreover, we describe with detail the antimicrobial action of AMPs on two major parasitical infections: leishmaniasis and malaria. All the features reviewed here show that AMPs are promising drugs to target protozoan parasites and that further understanding of the mechanism of action of these compounds will lead to improved drugs that could be worth to test in a clinical phase.

Antimicrobial peptides (AMPs) are important effectors of the innate immune system and play a vital role in the prevention of infections. Due to the increased emergence of new antibiotic-resistant bacteria, new drugs are constantly under investigation. AMPs in particular are recognized as promising candidates because of their modularity and wide antimicrobial spectrum. However, the mechanisms of action of AMPs, as well as their structure-activity relationships, are not completely understood. AMPs display no conserved three-dimensional structure and poor sequence conservation, which hinders rational design. Several bioinformatics tools have been developed to generate new templates with appealing antimicrobial properties with the aim of finding highly active peptide compounds with low cytotoxicity. The current tools reviewed here allow for the prediction and design of new active peptides with reasonable accuracy. However, a reliable method to assess the antimicrobial activity of AMPs has not yet been developed. The standardization of procedures to experimentally evaluate the antimicrobial activity of AMPs, together with the constant growth of current well-established databases, may allow for the future development of new bioinformatics tools to accurately predict antimicrobial activity.

Antimicrobial peptide drugs are increasingly attractive therapeutic agents as their roles in physiopathological processes are being unraveled and because the development of recombinant DNA technology has made them economically affordable in large amounts and high purity. However, due to lack of specificity regarding the target cells, difficulty in attaining them, or reduced half-lives, most current administration methods require high doses. On the other hand, reduced specificity of toxic drugs demands low concentrations to minimize undesirable side-effects, thus incurring the risk of having sublethal amounts which favour the appearance of resistant microbial strains. In this scenario, targeted delivery can fulfill the objective of achieving the intake of total quantities sufficiently low to be innocuous for the patient but that locally are high enough to be lethal for the infectious agent. One of the major advances in recent years has been the size reduction of drug carriers that have dimensions in the nanometer scale and thus are much smaller than —and capable of being internalized by— many types of cells. Among the different types of potential antimicrobial peptide-encapsulating structures reviewed here are liposomes, dendritic polymers, solid core nanoparticles, carbon nanotubes, and DNA cages. These nanoparticulate systems can be functionalized with a plethora of biomolecules providing specificity of binding to particular cell types or locations; as examples of these targeting elements we will present antibodies, DNA aptamers, cellpenetrating peptides, and carbohydrates. Multifunctional Trojan horse-like nanovessels can be engineered by choosing the adequate peptide content, encapsulating structure, and targeting moiety for each particular application.

Antimicrobial peptides (AMP's) are small peptides that have evolved as part of an innate cell defense mechanism in many organisms. We are currently developing methodologies to use these molecules to control the transmission of vector borne diseases utilizing a paratransgenic strategy. In this approach, symbiotic or commensal microbes of host insects are transformed to express gene products that interfere with pathogen transmission. These genetically altered microbes are re-introduced back to the insect where expression of the engineered molecules decreases the host's ability to transmit the pathogen. In previous work, we demonstrated that the paratransgenic expression of the AMP, cecropin A, by transformed microbes residing in the midgut of the reduviid bug, reduced carriage of the parasite, T. cruzi, substantially. In more recent work, we reported a dramatic increase in parasite killing efficiency when AMP's are used in combination. Further, the AMP concentrations required for parasite killing are decreased by at least 10-fold. In this review, we discuss the feasibility of utilizing other AMP's, individually or in combination, as effector molecules to control the transmission of leishmania parasites by sand flies and to control Vibriosis, a highly devastating disease in shrimp mariculture.

Pantothenate kinase-associated neurodegeneration (PKAN) is a hereditary progressive disorder and the most frequent form of neurodegeneration with brain iron accumulation (NBIA). PKAN patients present with a progressive movement disorder, dysarthria, cognitive impairment and retinitis pigmentosa. In magnetic resonance imaging, PKAN patients exhibit the pathognonomic “eye of the tiger” sign in the globus pallidus which corresponds to iron accumulation and gliosis as shown in neuropathological examinations. The discovery of the disease causing mutations in PANK2 has linked the disorder to coenzyme A (CoA) metabolism. PANK2 is the only one out of four PANK genes encoding an isoform which localizes to mitochondria. At least two other NBIA genes (PLA2G6, C19orf12) encode proteins that share with PANK2 a mitochondrial localization and all are suggested to play a role in lipid homeostasis. With no causal therapy available for PKAN until now, only symptomatic treatment is possible. A multi-centre retrospective study with bilateral pallidal deep brain stimulation in patients with NBIA revealed a significant improvement of dystonia. Recently, studies in the PANK Drosophila model “fumble” revealed improvement by the compound pantethine which is hypothesized to feed an alternate CoA biosynthesis pathway. In addition, pilot studies with the iron chelator deferiprone that crosses the blood brain barrier showed a good safety profile and some indication of efficacy. An adequately powered randomized clinical trial will start in 2012. This review summarizes clinical presentation, neuropathology and pathogenesis of PKAN.

Ceruloplasmin contains 95% of the copper in human serum and plays an important role in iron efflux from mammalian cells, including brain cells, due to the activity of ferroxidase, which oxidizes ferrous iron following its transfer to the cell surface via the iron transporter, ferroportin, and delivers ferric iron to extracellular transferrin. In the central nervous system, a glycosylphosphatidylinositol (GPI)-anchored ceruloplasmin bound to the cell membranes of astrocytes was found to be the major isoform of this protein. Inherited loss of the protein causes aceruloplasminemia, which is an autosomal recessive disorder characterized by progressive neurodegeneration of the retina and basal ganglia associated with specific inherited mutations in the ceruloplasmin gene. Aceruloplasminemia is classified as an inherited neurodegenerative disorder called “neurodegeneration with brain iron accumulation” (NBIA) due to genetic defects associated with iron metabolism. Clinical and pathologic studies in patients with aceruloplasminemia and ceruloplasmin knockout mice revealed increased lipid peroxidation due to iron-mediated cellular radical injury which is caused by a marked accumulation of iron in the affected parenchymal tissues such as the retina, liver, pancreas and brain. In the following review of aceruloplasminemia, the ceruloplasmin gene expression, structure and function will be presented, and the role of ceruloplasmin in iron metabolism will be discussed. The pathogenesis of aceruloplasminemia provides valuable insights into the mechanisms regulating iron homeostasis and also identified models that can be used to further dissect the role of this metal in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases, in which iron is accumulated.

Neuroferritinopathy is an autosomal dominant extra – pyramidal movement disorder caused by mutations in the ferritin light chain gene (FTL). The most frequent presentation is with chorea (50%), followed by dystonia (42.5 %) and parkinsonism (7.5%). Seven different mutations are known; 6 insertions in exon 4 and a missense mutation in exon 3 with the 460insA mutation in exon 4 being the most common. Brain magnetic resonance imaging demonstrates iron deposition in the basal ganglia and cavitation. Neuropathological studies have shown neuronal loss in the cerebral cortex, cerebellum and basal ganglia. Ferritin inclusion bodies were demonstrated within neurons and glia. Studies of patient derived fibroblasts and HeLa cells expressing mutant ferritin demonstrate increased iron levels and oxidative stress. These abnormalities have been recapitulated in mouse models of neuroferritinopathy. There is no disease modifying treatement for neuroferritinopathy but benzodiazepines and botulinum toxin may palliate dystonia and tetrabenazine may relieve chorea and facial tics. There is no role for iron chelation.

There is a wide variety of genetic and sporadic causes for neurodegenerative disorders with apparent brain iron accumulation on magnetic resonance imaging. Rare recessive causes include PLA2G6 mutations (infantile neuroaxonal dystrophy), and mutations of ATP13A2 (Kufor Rakeb syndrome) and FA2H. A variety of sporadic neurological disorders can present brain iron accumulation on imaging, including multiple sclerosis and neurological manifestations of HIV infection. The relevant clinical and imaging features will be discussed.

In a brief overview, in NO–sGC–cGMP signaling in a blood vessel, l-arginine is converted in the endothelium monolayer by the endothelial nitric oxide synthase (eNOS) to NO which diffuses into both the vessel lumen and the vessel wall, thereby activating soluble guanylate cyclase (sGC). Heme-dependent sGC stimulators and hem-independent sGC activators increase the cellular cGMP concentration via the direct activation of sGC, which results in both vasorelaxation and inhibition of platelet aggregation. Studies of the 90´s definitively established the role of endothelium in all cardiovascular diseases, which were associated with endothelial dysfunction by impaired release of endothelium-derived relaxing factors with consequent risk of spasm and thrombosis. The rationale of this review is based on the fact that the discovery of NO changed the concepts of cardiovascular disease mechanisms. However, considering the jargon "from the bench to clinical practice" we concluded that a potential therapeutic revolution did not follow the pathophysiological revolution. The review is focused on general aspects without regard for advanced research aspects, and designed in two main groups: the NO/cGMP positive stimulators and blockers as “future and encouraging” new therapeutic drugs. The potential vasodilators include 1) NOS uncoupling; 2) NOS enhancers (AVE compounds); 3) NO donors (nitrovasodilators); 4) NOindependent activators (BAY compounds), and; 5) PDE5 inhibitors. The potential vasoconstrictors include 1) NOSblockers (L-NAME, L-NMMA); 2) sGC-blockers (methylene blue), and; 3) PDEs. Few texts, selected by excellence and relevance, were crucial and considerably facilitated the elaboration of this text, in addition to our own experimental and clinical experience working on vasoplegic endothelium dysfunction.

This review focuses on TNF-related apoptosis-inducing ligand (TRAIL), also called Apo2 ligand, a protein belonging to the TNF superfamily. TRAIL can be found either in its transmembrane or circulating form, and its mostly studied peripheral effect is the induction of cellular apoptosis. Here, we discuss the evidences supporting the use of TRAIL as biomarker of cardiovascular diseases as well as the evidences showing the potential beneficial therapeutic effects of TRAIL on cardiovascular diseases and diabetes.