Coincident exposure of phosphatidylethanolamine and anionic phospholipids on the surface of irradiated cells

Biomembrane lipids: When physics and chemistry join to shape biological activity

Biomembranes constitute the first lines of defense of cells. While small molecules can often permeate cell walls in bacteria and plants, they are generally unable to penetrate the barrier constituted by the double layer of phospholipids, unless specific receptors or channels are present. Antimicrobial or cell-penetrating peptides are in fact highly specialized molecules able to bypass this barrier and even discriminate among different cell types. This capacity is made possible by the intrinsic properties of its phospholipids, their distribution between the internal and external leaflet, and their ability to mutually interact, modulating the membrane fluidity and the exposition of key headgroups. Although common phospholipids can be found in the membranes of most organisms, some are characteristic of specific cell types. Here, we review the properties of the most common lipids and describe how they interact with each other in biomembrane. We then discuss how their assembly in bilayers determines some key physical-chemical properties such as permeability, potential and phase status. Finally, we describe how the exposition of specific phospholipids determines the recognition of cell types by membrane-targeting molecules.

Pivotal roles for membrane phospholipids in axonal degeneration

Membrane phospholipids are critical components of several signaling pathways. Maintained in a variety of asymmetric distributions, their trafficking across the membrane can be induced by intra-, extra-, and intercellular events. A familiar example is the externalization of phosphatidylserine from the inner leaflet to the outer leaflet in apoptosis, inducing phagocytosis of the soma. Recently, it has been recognized that phospholipids in the axonal membrane may be a signal for axonal degeneration, regeneration, or other processes. This review focuses on key recent developments and areas for ongoing investigations. KEY FACTS: Phosphatidylserine externalization propagates along an axon after axonal injury and is delayed in the Wallerian degeneration slow (Wld^S) mutant. The ATP8A2 flippase mutant has spontaneous axonal degeneration. Microdomains of axonal degeneration in spheroid bodies have differential externalization of phosphatidylserine and phosphatidylethanolamine. Phospholipid trafficking could represent a mechanism for coordinated axonal degeneration and elimination, i.e. axoptosis, analogous to apoptosis of the cell body.

Development of Duramycin-Based Molecular Probes for Cell Death Imaging

Cell death is involved in numerous pathological conditions such as cardiovascular disorders, ischemic stroke and organ transplant rejection, and plays a critical role in the treatment of cancer. Cell death imaging can serve as a noninvasive means to detect the severity of tissue damage, monitor the progression of diseases, and evaluate the effectiveness of treatments, which help to provide prognostic information and guide the formulation of individualized treatment plans. The high abundance of phosphatidylethanolamine (PE), which is predominantly confined to the inner leaflet of the lipid bilayer membrane in healthy mammalian cells, becomes exposed on the cell surface in the early stages of apoptosis or accessible to the extracellular milieu when the cell suffers from necrosis, thus representing an attractive target for cell death imaging. Duramycin is a tetracyclic polypeptide that contains 19 amino acids and can bind to PE with excellent affinity and specificity. Additionally, this peptide has several favorable structural traits including relatively low molecular weight, stability to enzymatic hydrolysis, and ease of conjugation and labeling. All these highlight the potential of duramycin as a candidate ligand for developing PE-specific molecular probes. By far, a couple of duramycin-based molecular probes such as Tc-99 m-, F-18-, or Ga-68-labeled duramycin have been developed to target exposed PE for in vivo noninvasive imaging of cell death in different animal models. In this review article, we describe the state of the art with respect to in vivo imaging of cell death using duramycin-based molecular probes, as validated by immunohistopathology.

16pdel lipid changes in iPSC-derived neurons and function of FAM57B in lipid metabolism and synaptogenesis

The complex 16p11.2 deletion syndrome (16pdel) is accompanied by neurological disorders, including epilepsy, autism spectrum disorder, and intellectual disability. We demonstrated that 16pdel iPSC differentiated neurons from affected people show augmented local field potential activity and altered ceramide-related lipid species relative to unaffected. FAM57B, a poorly characterized gene in the 16p11.2 interval, has emerged as a candidate tied to symptomatology. We found that FAM57B modulates ceramide synthase (CerS) activity, but is not a CerS per se. In FAM57B mutant human neuronal cells and zebrafish brain, composition and levels of sphingolipids and glycerolipids associated with cellular membranes are disrupted. Consistently, we observed aberrant plasma membrane architecture and synaptic protein mislocalization, which were accompanied by depressed brain and behavioral activity. Together, these results suggest that haploinsufficiency of FAM57B contributes to changes in neuronal activity and function in 16pdel syndrome through a crucial role for the gene in lipid metabolism.

Constitutive oxidants from hepatocytes of male iPLA2β-null mice increases the externalization of phosphatidylethanolamine on plasma membrane

We have found that group VIA calcium-independent phospholipase A2 (iPLA2β) has specificity for hydrolysis of phosphatidylethanolamine (PE) in mouse livers. Phospholipids (PLs) are transported to plasma membrane and some PLs including PE are externalized to maintain membrane PL asymmetry. Here we demonstrated that hepatocytes of iPLA2β-null (KO) mice showed an increase in PE containing palmitate and oleate. We aimed to examine whether externalization of PE on the outer leaflets could be affected by iPLA2β deficiency and its modulation by reactive oxygen species (ROS) or apoptosis. As duramycin has high affinity to PE, we used duramycin conjugated with biotin (DLB) and streptavidin 488 as a probe for detection of externalized PE. Compared to WT, naïve KO hepatocytes showed an increase in both PE externalization and ROS generation. These events were observed in male but not in female KO mice. Hydrogen peroxide or menadione treatment enhanced PE externalization to the same extent for both male/female WT and KO hepatocytes. By indirect immunofluorescence, DLB-streptavidin staining was observed as small punctuated spots on the cell surface of menadione-treated KO hepatocytes. Unlike the reported PS externalization, CD95/FasL treatment did not lead to any increase in PE externalization, and iPLA2β deficiency-dependent PE externalization was also not correlated with apoptosis. Thus, constitutive (but not induced) ROS generation in iPLA2β-deficient hepatocytes leads to PE externalization observed only in male mice. Such PE externalization may imply detrimental effects regarding further oxidation of PE fatty acids and the binding with pathogens on the outer leaflets of hepatocyte plasma membrane.

Axonal degeneration induces distinct patterns of phosphatidylserine and phosphatidylethanolamine externalization

Axonal degeneration is a common feature of multiple neurodegenerative diseases, yet the mechanisms underlying its various manifestations are incompletely understood. We previously demonstrated that axonal degeneration is associated with externalization of phosphatidylserine (PS), which precedes morphological evidence of degeneration, is redox-sensitive, and is delayed in Wallerian degeneration slow (Wld^S) mutant animals. Phosphatidylethanolamine (PE) is the other major membrane phospholipid in the inner leaflet of the cell membrane, and given that PS signals apoptosis, phagocytosis, and degeneration, we hypothesized that PS and PE membrane dynamics play distinct roles in axonal degeneration. To test this hypothesis, axonal degeneration was induced with calcium ionophores in postnatal rat retinal ganglion cells, and PS- and PE-specific fluorescent probes used to measure their externalization over time. In untreated cells, cell-surface PS was prominent in the cell body alone. Elevation of intracellular calcium with calcium ionophores resulted in significantly increased levels of PS externalization in the cell body, axon, and axon growth cone. Unlike PS, cell-surface PE was diffusely distributed in untreated cells, with comparable levels across the soma, axons, and axon terminals. After exposure to calcium ionophores, PE externalization significantly increased in the cell body and axon. Elevated intracellular calcium also resulted in the formation of axonal blebs which exclusively contained externalized PS, but not PE. Together, these results indicated distinct patterns of externalized PS and PE in normal and degenerating neurons, suggesting a differential role for these phospholipids in transducing neuronal injury.

Ionization properties of monophosphoinositides in mixed model membranes

Phosphoinositides are found in low concentration in cellular membranes but perform numerous functions such as signaling, membrane trafficking, protein recruitment and modulation of protein activity. Spatiotemporal regulation by enzymes that phosphorylate and dephosphorylate the inositol ring results in the production of seven distinct and functionally diverse derivatives. Ionization properties of the phosphorylated headgroups of anionic lipids have been shown to impact how they interact with proteins and lipids in the membrane. While the ionization properties of the three bis and one tris phosphorylated forms have been studied in physiologically relevant model membranes, that of the monophosphorylated forms (i.e., phosphatidylinositol-3-phosphate (PI3P), phosphatidylinositol-4-phosphate (PI4P), phosphatidylinositol-5-phosphate (PI5P)) has received less attention. Here, we used ³¹P MAS NMR to determine the charge of 5 mol% of the monophosphorylated derivatives in pure dioleoylphosphatidylcholine (DOPC) and DOPC/dioleoylphosphatidylethanolamine (DOPE) bilayers as a function of pH. We find that PI3P, PI4P and PI5P each have unique pKa2 values in a DOPC bilayer, and each is reduced in DOPC/DOPE model membranes through the interaction of their headgroups with DOPE according to the electrostatic-hydrogen bond switch model. In this study, using model membranes mimicking the plasma membrane (inner leaflet), Golgi, nuclear membrane, and endosome (outer leaflet), we show that PI3P, PI4P or PI5P maximize their charge at neutral pH. Our results shed light on the electrostatics of the monophosphorylated headgroups of PI3P, PI4P, and PI5P and form the basis of their intracellular functions.

Molecular Basis of the Anticancer and Antibacterial Properties of CecropinXJ Peptide: An In Silico Study

Esophageal cancer is an aggressive lethal malignancy causing thousands of deaths every year. While current treatments have poor outcomes, cecropinXJ (CXJ) is one of the very few peptides with demonstrated in vivo activity. The great interest in CXJ stems from its low toxicity and additional activity against most ESKAPE bacteria and fungi. Here, we present the first study of its mechanism of action based on molecular dynamics (MD) simulations and sequence-property alignment. Although unstructured in solution, predictions highlight the presence of two helices separated by a flexible hinge containing P24 and stabilized by the interaction of W2 with target biomembranes: an amphipathic helix-I and a poorly structured helix-II. Both MD and sequence-property alignment point to the important role of helix I in both the activity and the interaction with biomembranes. MD reveals that CXJ interacts mainly with phosphatidylserine (PS) but also with phosphatidylethanolamine (PE) headgroups, both found in the outer leaflet of cancer cells, while salt bridges with phosphate moieties are prevalent in bacterial biomimetic membranes composed of PE, phosphatidylglycerol (PG) and cardiolipin (CL). The antibacterial activity of CXJ might also explain its interaction with mitochondria, whose phospholipid composition recalls that of bacteria and its capability to induce apoptosis in cancer cells.

Common and Differential Traits of the Membrane Lipidome of Colon Cancer Cell Lines and their Secreted Vesicles: Impact on Studies Using Cell Lines

Colorectal cancer (CRC) is the fourth leading cause of cancer death in the world. Despite the screening programs, its incidence in the population below the 50s is increasing. Therefore, new stratification protocols based on multiparametric approaches are highly needed. In this scenario, the lipidome is emerging as a powerful tool to classify tumors, including CRC, wherein it has proven to be highly sensitive to cell malignization. Hence, the possibility to describe the lipidome at the level of lipid species has renewed the interest to investigate the role of specific lipid species in pathologic mechanisms, being commercial cell lines, a model still heavily used for this purpose. Herein, we characterize the membrane lipidome of five commercial colon cell lines and their extracellular vesicles (EVs). The results demonstrate that both cell and EVs lipidome was able to segregate cells according to their malignancy. Furthermore, all CRC lines shared a specific and strikingly homogenous impact on ether lipid species. Finally, this study also cautions about the need of being aware of the singularities of each cell line at the level of lipid species. Altogether, this study firmly lays the groundwork of using the lipidome as a solid source of tumor biomarkers.

Recognition Sites for Cancer-targeting Drug Delivery Systems

BACKGROUND

Target-homing drug delivery systems are now gaining significant attention for use as novel therapeutic approaches in antitumor targeting for cancer therapy. Numerous targeted drug delivery systems have been designed to improve the targeting effects because these systems can display a range of favorable properties, thus, providing suitable characteristics for clinical applicability of anticancer drugs, such as increasing the solubility, and improving the drug distribution at target sites. The majority of these targeting systems are designed with respect to differences between cancerous and normal tissues, for instance, the low pH of tumor tissues or overexpressed receptors on tumor cell membranes. Due to the growing number of targeting possibilities, it is important to know the tumor-specific recognition strategies for designing novel, targeted, drug delivery systems. Herein, we identify and summarize literature pertaining to various recognition sites for optimizing the design of targeted drug delivery systems to augment current chemotherapeutic approaches.

OBJECTIVE

This review focuses on the identification of the recognition sites for developing targeted drug delivery systems for use in cancer therapeutics.

METHODS

We have reviewed and compiled cancer-specific recognition sites and their abnormal characteristics within tumor tissues (low pH, high glutathione, targetable receptors, etc.), tumor cells (receptor overexpression or tumor cell membrane changes) and tumor cell organelles (nuclear and endoplasmic reticular dysregulation) utilizing existing scientific literature. Moreover, we have highlighted the design of some targeted drug delivery systems that can be used as homing tools for these recognition sites.

RESULTS AND CONCLUSION

Targeted drug delivery systems are a promising therapeutic approach for tumor chemotherapy. Additional research focused on finding novel recognition sites, and subsequent development of targeting moieties for use with drug delivery systems will aid in the evaluation and clinical application of new and improved chemotherapeutics.

Imaging Cell Death: Focus on Early Evaluation of Tumor Response to Therapy

Cell death plays a prominent role in the treatment of cancer, because most anticancer therapies act by the induction of cell death including apoptosis, necrosis, and other pathways of cell death. Imaging cell death helps to identify treatment responders from nonresponders and thus enables patient-tailored therapy, which will increase the likelihood of treatment response and ultimately lead to improved patient survival. By taking advantage of molecular probes that specifically target the biomarkers/biochemical processes of cell death, cell death imaging can be successfully achieved. In recent years, with the increased understanding of the molecular mechanism of cell death, a variety of well-defined biomarkers/biochemical processes of cell death have been identified. By targeting these established cell death biomarkers/biochemical processes, a set of molecular imaging probes have been developed and evaluated for early monitoring treatment response in tumors. In this review, we mainly present the recent advances in identifying useful biomarkers/biochemical processes for both apoptosis and necrosis imaging and in developing molecular imaging probes targeting these biomarkers/biochemical processes, with a focus on their application in early evaluation of tumor response to therapy.

Occlusion of Animal Model Arteriovenous Malformations Using Vascular Targeting

Brain arteriovenous malformations (AVMs) are a significant cause of intracerebral hemorrhage in children and young adults. Currently, one third of patients have no viable treatment options. Vascular targeting agents (VTAs) are being designed to deliver pro-thrombotic molecules to the abnormal AVM vessels for rapid occlusion and cure. This study assessed the efficacy of a pro-thrombotic VTA targeting phosphatidylserine (PS) in a radiation-primed AVM animal model. The model AVM was surgically created in rats by anastomosis of the left external jugular vein to the adjacent common carotid artery. After 6 weeks, the AVM was irradiated (20 Gy) using gamma knife surgery (GKS). A PS-targeting VTA was created by conjugation of annexin V with human thrombin and administered intravenously 3 weeks post-GKS or sham. Unconjugated thrombin was used as a non-targeting control. AVM thrombosis and occlusion was monitored 3 weeks later by angiography and histology. Preliminary experiments established a safe dose of active thrombin for systemic administration. Subsequently, a single dose of annexin V-thrombin conjugate (0.77 mg/kg) resulted in angiographic AVM occlusion in sham (75%) and irradiated (63%) animals, while non-targeted thrombin did not. Lowering the conjugate dose (0.38 mg/kg) decreased angiographic AVM occlusion in sham (13%) relative to irradiated (80%) animals (p = 0.03) as did delivery of two consecutive doses of 0.38 mg/kg, 2 days apart (sham (0%); irradiated (78%); p = 0.003). These findings demonstrate efficacy of the PS-targeting VTA and the feasibility of a vascular targeting approach for occlusion of high-flow AVMs. Targeting specificity can be enhanced by radiation-sensitization and VTA dose modification.

Molecular determinants as therapeutic targets in cancer chemotherapy: An update

It is well known that cancer cells are heterogeneous in nature and very distinct from their normal counterparts. Commonly these cancer cells possess different and complementary metabolic profile, microenvironment and adopting behaviors to generate more ATPs to fulfill the requirement of high energy that is further utilized in the production of proteins and other essentials required for cell survival, growth, and proliferation. These differences create many challenges in cancer treatments. On the contrary, such situations of metabolic differences between cancer and normal cells may be expected a promising strategy for treatment purpose. In this article, we focus on the molecular determinants of oncogene-specific sub-organelles such as potential metabolites of mitochondria (reactive oxygen species, apoptotic proteins, cytochrome c, caspase 9, caspase 3, etc.), endoplasmic reticulum (unfolded protein response, PKR-like ER kinase, C/EBP homologous protein, etc.), nucleus (nucleolar phosphoprotein, nuclear pore complex, nuclear localization signal), lysosome (microenvironment, etc.) and plasma membrane phospholipids, etc. that might be exploited for the targeted delivery of anti-cancer drugs for therapeutic benefits. This review will help to understand the various targets of subcellular organelles at molecular levels. In the future, this molecular level understanding may be combined with the genomic profile of cancer for the development of the molecularly guided or personalized therapeutics for complete eradication of cancer.

A Comparison of [99mTc]Duramycin and [99mTc]Annexin V in SPECT/CT Imaging Atherosclerotic Plaques

PURPOSE

Apoptosis is a key factor in unstable plaques. The aim of this study is to evaluate the utility of visualizing atherosclerotic plaques with radiolabeled duramycin and Annexin V.

PROCEDURES

ApoE^-/- mice were fed with a high-fat diet to develop atherosclerosis, C57 mice as a control. Using a routine conjugation protocol, highly pure [^99mTc]duramycin and [^99mTc]Annexin V were obtained, which were applied for in vitro cell assays of apoptosis and in vivo imaging of atherosclerotic plaques in the animal model. Oil Red O staining, TUNEL, hematoxylin-eosin (HE), and CD68 immunostaining were used to evaluate the deposition of lipids and presence of apoptotic macrophages in the lesions where focal intensity positively correlated with the uptake of both tracers.

RESULTS

[^99mTc]duramycin and [^99mTc]Annexin V with a high radiochemical purity (97.13 ± 1.52 and 94.94 ± 0.65 %, respectively) and a well stability at room temperature were used. Apoptotic cells binding activity to [^99mTc]duramycin (Kd, 6.92 nM and Bmax, 56.04 mol/10¹⁹ cells) was significantly greater than [^99mTc]Annexin V (Kd, 12.63 nM and Bmax, 31.55 mol/10¹⁹ cells). Compared with [^99mTc]Annexin V, [^99mTc]duramycin bound avidly to atherosclerotic lesions with a higher plaque-to-background ratio (P/B was 8.23 ± 0.91 and 5.45 ± 0.48 at 20 weeks, 15.02 ± 0.23 and 12.14 ± 0.22 at 30 weeks). No plaques were found in C57 control mice. Furthermore, Oil Red O staining showed lipid deposition areas were significantly increased in ApoE^-/- mice at 20 and 30 weeks, and TUNEL and CD68 staining confirmed that the focal uptake of both tracers contained abundant apoptotic macrophages.

CONCLUSIONS

This stable, fast clearing, and highly specific [^99mTc]duramycin, therefore, can be useful for the quantification of vulnerable atherosclerotic plaques.

Identification of lipid-phosphatidylserine (PS) as the target of unbiasedly selected cancer specific peptide-peptoid hybrid PPS1

Phosphatidylserine (PS) is an anionic phospholipid maintained on the inner-leaflet of the cell membrane and is externalized in malignant cells. We previously launched a careful unbiased selection targeting biomolecules (e.g. protein, lipid or carbohydrate) distinct to cancer cells by exploiting HCC4017 lung cancer and HBEC30KT normal epithelial cells derived from the same patient, identifying HCC4017 specific peptide-peptoid hybrid PPS1. In this current study, we identified PS as the target of PPS1. We validated direct PPS1 binding to PS using ELISA-like assays, lipid dot blot and liposome based binding assays. In addition, PPS1 recognized other negatively charged and cancer specific lipids such as phosphatidic acid, phosphatidylinositol and phosphatidylglycerol. PPS1 did not bind to neutral lipids such as phosphatidylethanolamine found in cancer and phosphatidylcholine and sphingomyelin found in normal cells. Further we found that the dimeric version of PPS1 (PPS1D1) displayed strong cytotoxicity towards lung cancer cell lines that externalize PS, but not normal cells. PPS1D1 showed potent single agent anti-tumor activity and enhanced the efficacy of docetaxel in mice bearing H460 lung cancer xenografts. Since PS and anionic phospholipid externalization is common across many cancer types, PPS1 may be an alternative to overcome limitations of protein targeted agents.

Ethanolamine and Phosphatidylethanolamine: Partners in Health and Disease

Phosphatidylethanolamine (PE) is the second most abundant phospholipid in mammalian cells. PE comprises about 15–25% of the total lipid in mammalian cells; it is enriched in the inner leaflet of membranes, and it is especially abundant in the inner mitochondrial membrane. PE has quite remarkable activities: it is a lipid chaperone that assists in the folding of certain membrane proteins, it is required for the activity of several of the respiratory complexes, and it plays a key role in the initiation of autophagy. In this review, we focus on PE's roles in lipid-induced stress in the endoplasmic reticulum (ER), Parkinson's disease (PD), ferroptosis, and cancer.

Phosphatidylserine Translocation after Radiosurgery in an Animal Model of Arteriovenous Malformation

Phosphatidylserine (PS) is asymmetrically distributed across the plasma membrane, located predominantly on the inner leaflet in healthy cells. Translocation of PS to the outer leaflet makes it available as a target for biological therapies. We examined PS translocation after radiosurgery in an animal model of brain arteriovenous malformation (AVM). An arteriovenous fistula was created by end-to-side anastomosis of the left external jugular vein to the common carotid artery in 6-week-old, male Sprague Dawley rats. Six weeks after AVM creation, 15 rats underwent Gamma Knife stereotactic radiosurgery receiving a single 15 Gy dose to the margin of the fistula; 15 rats received sham treatment. Externalization of PS was examined by intravenous injection of a PS-specific near-infrared probe, PSVue-794, and in vivo fluorescence optical imaging at 1, 7, 21, 42, 63 and 84 days postirradiation. Fluorescent signaling indicative of PS translocation to the luminal cell surface accumulated in the AVM region, in both irradiated and nonirradiated animals, at all time points. Fluorescence was localized specifically to the AVM region and was not present in any other anatomical sites. Translocated PS increased over time in irradiated rats (P < 0.001) but not in sham-irradiated rats and this difference reached statistical significance at day 84 (P < 0.05). In summary, vessels within the mature rat AVM demonstrate elevated PS externalization compared to normal vessels. A single dose of ionizing radiation can increase PS externalization in a time-dependent manner. Strict localization of PS externalization within the AVM region suggests that stereotactic radiosurgery can serve as an effective priming agent and PS may be a suitable candidate for vascular-targeting approaches to AVM treatment.

Nanoparticle design considerations for molecular imaging of apoptosis: Diagnostic, prognostic, and therapeutic value

The present review analyzes various approaches for the design and synthesis of different nanoparticles for imaging and therapy. Nanoparticles for computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET) and optical imaging are discussed. The influence of nanoparticle size, shape, surface charge, composition, surface functionalization, active targeting and other factors on imaging and therapeutic efficacy is analyzed. Cyto- and genotoxicity of nanoparticles are also discussed. Special attention in the review is paid to the imaging of apoptotic tissues and cells in different diseases.

Detection of atherosclerotic plaques in ApoE-deficient mice using (99m)Tc-duramycin

Apoptosis of macrophages and smooth muscle cells is linked to atherosclerotic plaque destabilization. The apoptotic cascade leads to exposure of phosphatidylethanolamine (PE) on the outer leaflet of the cell membrane, thereby making apoptosis detectable using probes targeting PE. The objective of this study was to exploit capabilities of a PE-specific imaging probe, (99m)Tc-duramycin, in localizing atherosclerotic plaque and assessing plaque evolution in apolipoprotein-E knockout (ApoE(-/-)) mice.

METHODS

Atherosclerosis was induced in ApoE(-/-) mice by feeding an atherogenic diet. (99m)Tc-duramycin images were acquired using a small-animal SPECT imager. Six ApoE(-/-) mice at 20weeks of age (Group I) were imaged and then sacrificed for ex vivo analyses. Six additional ApoE(-/-) mice (Group II) were imaged at 20 and 40weeks of age before sacrifice. Six ApoE wild-type (ApoE(+/+)) mice (Group III) were imaged at 40weeks as controls. Five additional ApoE(-/-) mice (40weeks of age) (Group IV) were imaged with a (99m)Tc-labeled inactive peptide, (99m)Tc-LinDUR, to assess (99m)Tc-duramycin targeting specificity.

RESULTS

Focal (99m)Tc-duramycin uptake in the ascending aorta and aortic arch was detected at 20 and 40weeks in the ApoE(-/-) mice but not in ApoE(+/+) mice. (99m)Tc-duramycin uptake in the aortic lesions increased 2.2-fold on quantitative imaging in the ApoE(-/-) mice between 20 and 40weeks. Autoradiographic and histological data indicated significantly increased (99m)Tc-duramycin uptake in the ascending aorta and aortic arch associated with advanced plaques. Quantitative autoradiography showed that the ratio of activity in the aortic arch to descending thoracic aorta, which had no plaques or radioactive uptake, was 2.1 times higher at 40weeks than at 20weeks (6.62±0.89 vs. 3.18±0.29, P<0.01). There was barely detectable focal uptake of (99m)Tc-duramycin in the aortic arch of ApoE(+/+) mice. No detectable (99m)Tc-LinDUR uptake was observed in the aortas of ApoE(-/-) mice.

CONCLUSIONS

PE-targeting properties of (99m)Tc-duramycin in the atherosclerotic mouse aortas were noninvasively characterized. (99m)Tc-duramycin is promising in localizing advanced atherosclerotic plaques.

Mammalian models of chemically induced primary malignancies exploitable for imaging-based preclinical theragnostic research

Compared with transplanted tumor models or genetically engineered cancer models, chemically induced primary malignancies in experimental animals can mimic the clinical cancer progress from the early stage on. Cancer caused by chemical carcinogens generally develops through three phases namely initiation, promotion and progression. Based on different mechanisms, chemical carcinogens can be divided into genotoxic and non-genotoxic ones, or complete and incomplete ones, usually with an organ-specific property. Chemical carcinogens can be classified upon their origins such as environmental pollutants, cooked meat derived carcinogens, N-nitroso compounds, food additives, antineoplastic agents, naturally occurring substances and synthetic carcinogens, etc. Carcinogen-induced models of primary cancers can be used to evaluate the diagnostic/therapeutic effects of candidate drugs, investigate the biological influential factors, explore preventive measures for carcinogenicity, and better understand molecular mechanisms involved in tumor initiation, promotion and progression. Among commonly adopted cancer models, chemically induced primary malignancies in mammals have several advantages including the easy procedures, fruitful tumor generation and high analogy to clinical human primary cancers. However, in addition to the time-consuming process, the major drawback of chemical carcinogenesis for translational research is the difficulty in noninvasive tumor burden assessment in small animals. Like human cancers, tumors occur unpredictably also among animals in terms of timing, location and the number of lesions. Thanks to the availability of magnetic resonance imaging (MRI) with various advantages such as ionizing-free scanning, superb soft tissue contrast, multi-parametric information, and utility of diverse contrast agents, now a workable solution to this bottleneck problem is to apply MRI for noninvasive detection, diagnosis and therapeutic monitoring on those otherwise uncontrollable animal models with primary cancers. Moreover, it is foreseeable that the combined use of chemically induced primary cancer models and molecular imaging techniques may help to develop new anticancer diagnostics and therapeutics.

The increasing role of phosphatidylethanolamine as a lipid receptor in the action of host defence peptides

Host defence peptides (HDPs) are antimicrobial agents produced by organisms across the prokaryotic and eukaryotic kingdoms. Many prokaryotes produce HDPs, which utilise lipid and protein receptors in the membranes of bacterial competitors to facilitate their antibacterial action and thereby survive in their niche environment. As a major example, it is well established that cinnamycin and duramycins from Streptomyces have a high affinity for phosphatidylethanolamine (PE) and exhibit activity against other Gram-positive organisms, such as Bacillus. In contrast, although eukaryotic HDPs utilise membrane interactive mechanisms to facilitate their antimicrobial activity, the prevailing view has long been that these mechanisms do not involve membrane receptors. However, this view has been recently challenged by reports that a number of eukaryotic HDPs such as plant cyclotides also use PE as a receptor to promote their antimicrobial activities. Here, we review current understanding of the mechanisms that underpin the use of PE as a receptor in the antimicrobial and other biological actions of HDPs and describe medical and biotechnical uses of these peptides, which range from tumour imaging and detection to inclusion in topical microbicidal gels to prevent the sexual transmission of HIV.

Molecular imaging of apoptosis: from micro to macro

Apoptosis, or programmed cell death, is involved in numerous human conditions including neurodegenerative diseases, ischemic damage, autoimmune disorders and many types of cancer, and is often confused with other types of cell death. Therefore strategies that enable visualized detection of apoptosis would be of enormous benefit in the clinic for diagnosis, patient management, and development of new therapies. In recent years, improved understanding of the apoptotic machinery and progress in imaging modalities have provided opportunities for researchers to formulate microscopic and macroscopic imaging strategies based on well-defined molecular markers and/or physiological features. Correspondingly, a large collection of apoptosis imaging probes and approaches have been documented in preclinical and clinical studies. In this review, we mainly discuss microscopic imaging assays and macroscopic imaging probes, ranging in complexity from simple attachments of reporter moieties to proteins that interact with apoptotic biomarkers, to rationally designed probes that target biochemical changes. Their clinical translation will also be our focus.

Highly specific PET imaging of prostate tumors in mice with an iodine-124-labeled antibody fragment that targets phosphatidylserine

Phosphatidylserine (PS) is an attractive target for imaging agents that identify tumors and assess their response to therapy. PS is absent from the surface of most cell types, but becomes exposed on tumor cells and tumor vasculature in response to oxidative stresses in the tumor microenvironment and increases in response to therapy. To image exposed PS, we used a fully human PS-targeting antibody fragment, PGN635 F(ab’)_2, that binds to complexes of PS and β2-glycoprotein I. PGN635 F(ab’)₂ was labeled with the positron-emitting isotope iodine-124 (¹²⁴I) and the resulting probe was injected into nude mice bearing subcutaneous or orthotopic human PC3 prostate tumors. Biodistribution studies showed that ¹²⁴I-PGN635 F(ab’)₂ localized with remarkable specificity to the tumors with little uptake in other organs, including the liver and kidneys. Clear delineation of the tumors was achieved by PET 48 hours after injection. Radiation of the tumors with 15 Gy or systemic treatment of the mice with 10 mg/kg docetaxel increased localization in the tumors. Tumor-to-normal (T/N) ratios were inversely correlated with tumor growth measured over 28 days. These data indicate that ¹²⁴I-PGN635 F(ab’)₂ is a promising new imaging agent for predicting tumor response to therapy.

A combination of radiosurgery and soluble tissue factor enhances vascular targeting for experimental glioblastoma

Radiosurgery for glioblastoma is limited to the development of resistance, allowing tumor cells to survive and initiate tumor recurrence. Based on our previous work that coadministration of tissue factor and lipopolysaccharide following radiosurgery selectively induced thrombosis in cerebral arteriovenous malformations, achieving thrombosis of 69% of the capillaries and 39% of medium sized vessels, we hypothesized that a rapid and selective shutdown of the capillaries in glioblastoma vasculature would decrease the delivery of oxygen and nutrients, reducing tumor growth, preventing intracranial hypertension, and improving life expectancy. Glioblastoma was formed by implantation of GL261 cells into C57Bl/6 mouse brain. Mice were intravenously injected tissue factor, lipopolysaccharide, a combination of both, or placebo 24 hours after radiosurgery. Control mice received both agents after sham irradiation. Coadministration of tissue factor and lipopolysaccharide led to the formation of thrombi in up to 87 ± 8% of the capillaries and 46 ± 4% of medium sized vessels within glioblastoma. The survival rate of mice in this group was 80% versus no survivor in placebo controls 30 days after irradiation. Animal body weight increased with time in this group (r = 0.88, P = 0.0001). Thus, radiosurgery enhanced treatment with tissue factor, and lipopolysaccharide selectively induces thrombosis in glioblastoma vasculature, improving life expectancy.

TAM receptors in apoptotic cell clearance, autoimmunity, and cancer

Receptor tyrosine kinases, Tyro-3, Axl and Mer, collectively designated as TAM, are involved in the clearance of apoptotic cells. TAM ligands, Gas6 and Protein S, bind to the surfaces of apoptotic cells, and at the same time, interact directly with TAM expressed on phagocytes, impacting the engulfment and clearance of apoptotic cells and debris. The well-tuned and balanced actions of TAM may affect a variety of human pathologies including autoimmunity, retinal degeneration, and cancer. This article emphasizes some of the emerging findings and mechanistic insights into TAM functions that are clinically relevant and possibly therapeutically targeted.

Duramycin exhibits antiproliferative properties and induces apoptosis in tumour cells

Duramycin is a polypeptide that binds specifically to phosphatidylethanolamine (PE) on cell surfaces with high affinity, and has been shown to disrupt tumour cell surface-based coagulation and exhibit weak antimicrobial activity. The aim of the present study was to characterize the effect of duramycin on tumour cell proliferation and viability. Duramycin was used to detect phosphatidylethanolamine expression on cell lines by flow cytometry. Cells were cultured in the presence of duramycin and proliferation and cell viability assessed. Electron microscopy and confocal microscopy were utilized to investigate cell membrane structure after duramycin treatment. Pancreatic tumour cells were shown to express phosphatidylethanolamine on their cell surfaces by specific labelling with duramycin. Phosphatidylethanolamine expression was generally increased in apoptotic cells and more so in necrotic cells. Cells cultured in the presence of duramycin showed increasing levels of apoptosis and ultimately necrosis with increasing duramycin concentrations, and cell proliferation was reduced in a duramycin dose-dependent manner between 0.125 and 12.5 μmol/l. Tissue factor expression was also reduced when cells were cultured in the presence of duramycin. Cells imaged by electron microscopy were fragile, suggesting that membrane integrity was compromised by duramycin, although no obvious differences in membrane structure were observed by live cell confocal imaging. Duramycin induced apoptosis and exhibited antiproliferative and anticoagulant effects on pancreatic tumour cells, most probably by disrupting cell membrane structure and/or function.

Biomarkers and molecular probes for cell death imaging and targeted therapeutics

Cell death is a critically important biological process. Disruption of homeostasis, either by excessive or deficient cell death, is a hallmark of many pathological conditions. Recent research advances have greatly increased our molecular understanding of cell death and its role in a range of diseases and therapeutic treatments. Central to these ongoing research and clinical efforts is the need for imaging technologies that can locate and identify cell death in a wide array of in vitro and in vivo biomedical samples with varied spatiotemporal requirements. This review article summarizes community efforts over the past five years to identify useful biomarkers for dead and dying cells, and to develop molecular probes that target these biomarkers for optical, radionuclear, or magnetic resonance imaging. Apoptosis biomarkers are classified as either intracellular (caspase enzymes, mitochondrial membrane potential, cytosolic proteins) or extracellular (plasma membrane phospholipids, membrane potential, surface exposed histones). Necrosis, autophagy, and senescence biomarkers are described, as well as unexplored cell death biomarkers. The article discusses possible chemotherapeutic and theranostic strategies, and concludes with a summary of current challenges and expected eventual rewards of clinical cell death imaging.

Phosphatidylserine externalization and membrane blebbing are involved in the nonclassical export of FGF1

The mechanisms of nonclassical export of signal peptide-less proteins remain insufficiently understood. Here, we demonstrate that stress-induced unconventional export of FGF1, a potent and ubiquitously expressed mitogenic and proangiogenic protein, is associated with and dependent on the formation of membrane blebs and localized cell surface exposure of phosphatidylserine (PS). In addition, we found that the differentiation of promonocytic cells results in massive FGF1 release, which also correlates with membrane blebbing and exposure of PS. These findings indicate that the externalization of acidic phospholipids could be used as a pharmacological target to regulate the availability of FGF1 in the organism.

Forms, crosstalks, and the role of phospholipid biosynthesis in autophagy

Autophagy is a highly conserved cellular process occurring during periods of stress to ensure a cell's survival by recycling cytosolic constituents and making products that can be used in energy generation and other essential processes. Three major forms of autophagy exist according to the specific mechanism through which cytoplasmic material is transported to a lysosome. Chaperone-mediated autophagy is a highly selective form of autophagy that delivers specific proteins for lysosomal degradation. Microautophagy is a less selective form of autophagy that occurs through lysosomal membrane invaginations, forming tubes and directly engulfing cytoplasm. Finally, macroautophagy involves formation of new membrane bilayers (autophagosomes) that engulf cytosolic material and deliver it to lysosomes. This review provides new insights on the crosstalks between different forms of autophagy and the significance of bilayer-forming phospholipid synthesis in autophagosomal membrane formation.

Development of bavituximab, a vascular targeting agent with immune-modulating properties, for lung cancer treatment

Bavituximab is a chimeric monoclonal antibody directed against the membrane phospholipid phosphatidylserine. Phosphatidylserine exposure is increased on endothelial cells and apoptotic cancer cells in solid tumors, allowing tumor-specific targeting of bavituximab. Bavituximab binding results in tumor vessel occlusion and enhanced antitumor immunity. Preclinical investigations have demonstrated efficacy as monotherapy and in combination with other modalities against multiple cancer types. Phase I clinical trials of bavituximab monotherapy and in combination with chemotherapy in adults with refractory solid tumors have been completed. Phase II trials of bavituximab in combination with chemotherapy for the first- and second-line treatment of advanced non-small-cell lung cancer are currently ongoing. This article summarizes the preclinical development and clinical experience with bavituximab in non-small-cell lung cancer.

Lantibiotics as probes for phosphatidylethanolamine

Phosphatidylethanolamine (PE) is a major component in the mammalian plasma membrane. It is present mainly in the inner leaflet of the membrane bilayer in a viable, typical mammalian cell. However, accumulating evidence indicates that a number of biological events involve PE externalization. For instance, PE is concentrated at the surface of cleavage furrow between mitotic daughter cells and is correlated with the dynamics of contractile ring. In apoptotic cells, PE is exposed to the cell surface, thus providing a molecular marker for detection. In addition, PE is a cofactor in the anticoagulant mechanism, and a distinct distribution profile of PE has been documented at the blood-endothelium interface. These recent discoveries were made possible using PE-specific probes derived from duramycin and cinnamycin, which are members of type B lantibiotics. This review provides an account on the features of these PE-specific lantibiotics in the context of molecular probes for the characterization of PE on a cellular and tissue level. According to the existing data, PE is likely a versatile chemical species that plays a role in the regulation of defined biological and physiological activities. The utilities of lantibiotic-based molecular probes will help accelerate the characterization of PE as an abundant, yet elusive membrane component.

Transbilayer phospholipids molecular imaging

Nuclear medicine has become a key part of molecular imaging. In the present review article, we focus on the transbilayer phospholipids as exquisite targets for radiolabelled probes in molecular imaging. Asymmetry of phospholipid distribution is a characteristic of mammalian cell membranes. Phosphatidylcholine and sphyngomyelin cholinophospholipids are primarily located within the external leaflet of the cell membrane. Phosphatidylserine and phosphatidylethanolamine aminophospholipids, and also phosphatidylinositol are primarily located within the internal leaflet of the cell membrane. New radiolabelled tracers have been designed in preclinical and clinical research for PET-CT and SPECT-CT molecular imaging of transbilayer phospholipids.

Increased exposure of phosphatidylethanolamine on the surface of tumor vascular endothelium

We have previously shown that oxidative stress within the tumor microenvironment causes phosphatidylserine (PS) to redistribute from the inner to the outer membrane leaflet of the endothelial cells (EC) creating a highly specific marker for the tumor vasculature. Because the distribution of phosphatidylethanolamine (PE) and PS within the membrane is coregulated, we reasoned that PE would also be localized in the outer membrane leaflet of tumor EC. To demonstrate this, the PE-binding peptide duramycin was biotinylated and used to determine the distribution of PE on EC in vitro and in vivo. Exposure of cultured EC to hypoxia, acidity, reactive oxygen species, or irradiation resulted in the formation of membrane blebs that were intensely PE-positive. When biotinylated duramycin was intravenously injected into tumor-bearing mice, it preferentially localized to the luminal surface of the vascular endothelium. Depending on tumor type, 13% to 56% of the tumor vessels stained positive for PE. PE-positive vessels were observed in and around hypoxic regions of the tumor. With the exception of intertubular vessels of the kidney, normal vessels remained unstained. To test the potential of PE as a biomarker for imaging, duramycin was conjugated to the near-infrared fluorophore 800CW and used for optical imaging of RM-9 prostate carcinomas. The near-infrared probe was easily detected within tumors in live animals. These results show that PE, like PS, becomes exposed on tumor vascular endothelium of multiple types of tumors and holds promise as a biomarker for noninvasive imaging and drug targeting.

An orthotopic lung tumor model for image-guided microirradiation in rats

The purpose of this study was to develop a rat orthotopic lung tumor model with a solitary intrapulmonary nodule to study the effects of high-dose radiation. A549-Luc non-small cell lung cancer (NSCLC) cells were implanted into nude rats in the intercostal space between ribs 5 and 6 of the right lung. Bioluminescence and microcomputed tomography (CT) imaging were performed after implantation to confirm the presence of a solitary tumor and to monitor tumor growth. A device using image guidance for localization was developed to facilitate high-precision irradiation in small animals. A pilot irradiation study was performed, and response was assessed by bioluminescence imaging and immunohistochemistry. Radiation response was confirmed through serial bioluminescence imaging, and the strength of the bioluminescence signal was observed to be inversely proportional to dose. Response was also observed by the monoclonal antibody bavituximab, which binds to exposed lipid phosphatidylserine (PS) on tumor vessels. The ability to (1) reproducibly generate solitary tumor nodules in the rat lung, (2) identify and monitor tumor growth by bioluminescence imaging and CT imaging, (3) accurately target these tumors using high doses of radiation, and (4) demonstrate and quantify radiation response using bioluminescence imaging provides significant opportunity to probe the biological mechanisms of high-dose irradiation in preclinical settings.

Plasma phospholipid metabolic profiling and biomarkers of rats following radiation exposure based on liquid chromatography-mass spectrometry technique

Lipidomics, a prominent area of metabolomics, utilizes novel analytical methodologies to study the extensive classes of lipid molecules, changes in lipid metabolism and lipid-mediated signaling processes. In this paper, the phospholipid metabolic profiles changes and potential biomarker identification in the rats plasma after gamma-irradiation exposure were investigated by coupling high performance liquid chromatography-mass spectrometry technology to multivariate statistical analysis. Orthogonal partial least-squares to latent structures discriminate analysis (OPLS-DA) was used to find the potential plasma phospholipids biomarkers of rats for radiation exposure. According to the corresponding tandem mass spectrometric results, potential biomarkers were identified. After exposure to gamma-rays, phosphatidylethanolamine and phosphatidylserine showed a marked increase, and phosphatidylcholine, sphingomyelin and lysophosphatidylcholine followed the same trend, although their rise was not significant. The results suggested that radiated rats had a phospholipid metabolic abnormality, which could be an alternative way to assess the radiaton exposure. The biomarkers may involve a radiation-induced apoptosis pathway and represent a promising target for discovery new radioprotective drugs and radiosensitizers.

Targeting inside-out phosphatidylserine as a therapeutic strategy for viral diseases

Targeting exposed anionic phospholipids on a spectrum of virus-infected cells can protect against lethal virus infections in vivo.