Oxidized phospholipid species promote in vivo differential cx43 phosphorylation and vascular smooth muscle cell proliferation

Oxidized Phospholipids Regulate Tenocyte Function via Induction of Amphiregulin in Dendritic Cells

Inflammation is a driving force of tendinopathy. The oxidation of phospholipids by free radicals is a consequence of inflammatory reactions and is an important indicator of tissue damage. Here, we have studied the impact of oxidized phospholipids (OxPAPC) on the function of human tenocytes. We observed that treatment with OxPAPC did not alter the morphology, growth and capacity to produce collagen in healthy or diseased tenocytes. However, since OxPAPC is a known modulator of the function of immune cells, we analyzed whether OxPAPC-treated immune cells might influence the fate of tenocytes. Co-culture of tenocytes with immature, monocyte-derived dendritic cells treated with OxPAPC (Ox-DCs) was found to enhance the proliferation of tenocytes, particularly those from diseased tendons. Using transcriptional profiling of Ox-DCs, we identified amphiregulin (AREG), a ligand for EGFR, as a possible mediator of this proliferation enhancing effect, which we could confirm using recombinant AREG. Of note, diseased tenocytes were found to express higher levels of EGFR compared to tenocytes isolated from healthy donors and show a stronger proliferative response upon co-culture with Ox-DCs, as well as AREG treatment. In summary, we identify an AREG-EGFR axis as a mediator of a DC-tenocyte crosstalk, leading to increased tenocyte proliferation and possibly tendon regeneration.

Connexin 43 Phosphorylation: Implications in Multiple Diseases

Connexin 43 (Cx43) is most widely distributed in mammals, especially in the cardiovascular and nervous systems. Its phosphorylation state has been found to be regulated by the action of more than ten kinases and phosphatases, including mitogen-activated protein kinase/extracellular signaling and regulating kinase signaling. In addition, the phosphorylation status of different phosphorylation sites affects its own synthesis and assembly and the function of the gap junctions (GJs) to varying degrees. The phosphorylation of Cx43 can affect the permeability, electrical conductivity, and gating properties of GJs, thereby having various effects on intercellular communication and affecting physiological or pathological processes in vitro and in vivo. Therefore, clarifying the relationship between Cx43 phosphorylation and specific disease processes will help us better understand the disease. Based on the above clinical and preclinical findings, we present in this review the functional significance of Cx43 phosphorylation in multiple diseases and discuss the potential of Cx43 as a drug target in Cx43-related disease pathophysiology, with an emphasis on the importance of connexin 43 as an emerging therapeutic target in cardiac and neuroprotection.

Redox phospholipidomics discovers pro-ferroptotic death signals in A375 melanoma cells in vitro and in vivo

Growing cancer cells effectively evade most programs of regulated cell death, particularly apoptosis. This necessitates a search for alternative therapeutic modalities to cause cancer cell's demise, among them - ferroptosis. One of the obstacles to using pro-ferroptotic agents to treat cancer is the lack of adequate biomarkers of ferroptosis. Ferroptosis is accompanied by peroxidation of polyunsaturated species of phosphatidylethanolamine (PE) to hydroperoxy- (-OOH) derivatives, which act as death signals. We demonstrate that RSL3-induced death of A375 melanoma cells in vitro was fully preventable by ferrostatin-1, suggesting their high susceptibility to ferroptosis. Treatment of A375 cells with RSL3 caused a significant accumulation of PE-(18:0/20:4-OOH) and PE-(18:0/22:4-OOH), the biomarkers of ferroptosis, as well as oxidatively truncated products - PE-(18:0/hydroxy-8-oxo-oct-6-enoic acid (HOOA) and PC-(18:0/HOOA). A significant suppressive effect of RSL3 on melanoma growth was observed in vivo (utilizing a xenograft model of inoculation of GFP-labeled A375 cells into immune-deficient athymic nude mice). Redox phospholipidomics revealed elevated levels of 18:0/20:4-OOH in RSL3-treated group vs controls. In addition, PE-(18:0/20:4-OOH) species were identified as major contributors to the separation of control and RSL3-treated groups, with the highest variable importance in projection predictive score. Pearson correlation analysis revealed an association between tumor weight and contents of PE-(18:0/20:4-OOH) (r = -0.505), PE-18:0/HOOA (r = -0.547) and PE 16:0-HOOA (r = -0.503). Thus, LC-MS/MS based redox lipidomics is a sensitive and precise approach for the detection and characterization of phospholipid biomarkers of ferroptosis induced in cancer cells by radio- and chemotherapy.

Mechanism of oxidized phospholipid-related inflammatory response in vascular ageing

Vascular ageing is an important factor in the morbidity and mortality of the elderly. Atherosclerosis is a characteristic disease of vascular ageing, which is closely related to the enhancement of vascular inflammation. Phospholipid oxidation products are important factors in inducing cellular inflammation. Through interactions with vascular cells and immune cells, they regulate intracellular signaling pathways, activate the expression of various cytokines, and affect cell behavior, such as metabolic level, proliferation, apoptosis, etc. Intervention in lipid metabolism and anti-inflammation are the two key pathways of drugs for the treatment of atherosclerosis. This review aims to sort out the signaling pathway of oxidized phospholipids-induced inflammatory factors in vascular cells and immune cells and the mechanism leading to changes in cell behavior, and summarize the therapeutic targets in the inflammatory signaling pathway for the development of atherosclerosis drugs.

Role of Connexin 43 phosphorylation on Serine-368 by PKC in cardiac function and disease

Intercellular communication mediated by gap junction channels and hemichannels composed of Connexin 43 (Cx43) is vital for the propagation of electrical impulses through cardiomyocytes. The carboxyl terminal tail of Cx43 undergoes various post-translational modifications including phosphorylation of its Serine-368 (S368) residue. Protein Kinase C isozymes directly phosphorylate S368 to alter Cx43 function and stability through inducing conformational changes affecting channel permeability or promoting internalization and degradation to reduce intercellular communication between cardiomyocytes. Recent studies have implicated this PKC/Cx43-pS368 circuit in several cardiac-associated diseases. In this review, we describe the molecular and cellular basis of PKC-mediated Cx43 phosphorylation and discuss the implications of Cx43 S368 phosphorylation in the context of various cardiac diseases, such as cardiomyopathy, as well as the therapeutic potential of targeting this pathway.

Connexin 43 across the Vasculature: Gap Junctions and Beyond

Connexin 43 (Cx43) is essential to the function of the vasculature. Cx43 proteins form gap junctions that allow for the exchange of ions and molecules between vascular cells to facilitate cell-to-cell signaling and coordinate vasomotor activity. Cx43 also has intracellular signaling functions that influence vascular cell proliferation and migration. Cx43 is expressed in all vascular cell types, although its expression and function vary by vessel size and location. This includes expression in vascular smooth muscle cells (vSMC), endothelial cells (EC), and pericytes. Cx43 is thought to coordinate homocellular signaling within EC and vSMC. Cx43 gap junctions also function as conduits between different cell types (heterocellular signaling), between EC and vSMC at the myoendothelial junction, and between pericyte and EC in capillaries. Alterations in Cx43 expression, localization, and post-translational modification have been identified in vascular disease states, including atherosclerosis, hypertension, and diabetes. In this review, we discuss the current understanding of Cx43 localization and function in healthy and diseased blood vessels across all vascular beds.

Glycated serum albumin decreases connexin 43 phosphorylation in the corpus cavernosum

Background

Glycated serum albumin (GSA) is an early glycosylation product that participates in diabetic vascular complications. This study examined the role of GSA in early damage to the corpus cavernosum and the involved mechanisms.

Methods

Nine 8-week-old male Sprague-Dawley (SD) rats (250-300 g) were divided into the control (saline vehicle, n=3) and GSA (200 µg/kg, n=6) groups. The corpus cavernosum tissues were harvested. Phosphorylated and non-phosphorylated connexin 43 (Cx43), endothelial nitric oxide synthase (eNOS), phosphatidylinositol 3-kinase (PI3K), and serine-threonine kinase (Akt) were tested by immunohistochemistry and western blotting. Human umbilical vein endothelial cells (HUVECs) overexpressing Cx43 were used to analyze the Cx43 phosphorylation sites (S368, S262, Y265, S255, and S279/282) using western blotting.

Results

The expression of phosphorylated Cx43 in the penis was significantly lower in GSA-treated rats than in controls. The expression levels of p-Cx43, p-eNOS, p-PI3K, and p-Akt were significantly decreased in HUVECs exposed to GSA in dose- and time-dependent manners. The most significant impact on all four proteins was observed with 1 µg/mL of GSA for 12 h. Phosphorylation at the S368, S262, Y265, S255, and S279/282 sites of Cx43 was downregulated by GSA, and S368 was the most significantly suppressed phosphorylation site compared with the other sites.

Conclusions

GSA decreases the expression of p-Cx43 in the corpus cavernosum of rats. This effect might be also related to the decreased phosphorylation of p-eNOS, p-PI3K, and p-Akt, as well as by the downregulation of phosphorylation at the S368 site.

Cell-To-Cell Communication in the Resistance Vasculature

The arterial vasculature can be divided into large conduit arteries, intermediate contractile arteries, resistance arteries, arterioles, and capillaries. Resistance arteries and arterioles primarily function to control systemic blood pressure. The resistance arteries are composed of a layer of endothelial cells oriented parallel to the direction of blood flow, which are separated by a matrix layer termed the internal elastic lamina from several layers of smooth muscle cells oriented perpendicular to the direction of blood flow. Cells within the vessel walls communicate in a homocellular and heterocellular fashion to govern luminal diameter, arterial resistance, and blood pressure. At rest, potassium currents govern the basal state of endothelial and smooth muscle cells. Multiple stimuli can elicit rises in intracellular calcium levels in either endothelial cells or smooth muscle cells, sourced from intracellular stores such as the endoplasmic reticulum or the extracellular space. In general, activation of endothelial cells results in the production of a vasodilatory signal, usually in the form of nitric oxide or endothelial-derived hyperpolarization. Conversely, activation of smooth muscle cells results in a vasoconstriction response through smooth muscle cell contraction. © 2022 American Physiological Society. Compr Physiol 12: 1-35, 2022.

Oxidized alkyl phospholipids stimulate sodium transport in proximal tubules via a non-genomic PPARγ-dependent pathway

Oxidized phospholipids have been shown to exhibit pleiotropic effects in numerous biological contexts. For example, 1-O-hexadecyl-2-azelaoyl-sn-glycero-3-phosphocholine (azPC), an oxidized phospholipid formed from alkyl phosphatidylcholines, is a peroxisome proliferator–activated receptor gamma (PPARγ) nuclear receptor agonist. Although it has been reported that PPARγ agonists including thiazolidinediones can induce plasma volume expansion by enhancing renal sodium and water retention, the role of azPC in renal transport functions is unknown. In the present study, we investigated the effect of azPC on renal proximal tubule (PT) transport using isolated PTs and kidney cortex tissues and also investigated the effect of azPC on renal sodium handling in vivo. We showed using a microperfusion technique that azPC rapidly stimulated Na⁺/HCO₃⁻ cotransporter 1 (NBCe1) and luminal Na⁺/H⁺ exchanger (NHE) activities in a dose-dependent manner at submicromolar concentrations in isolated PTs from rats and humans. The rapid effects (within a few minutes) suggest that azPC activates NBCe1 and NHE via nongenomic signaling. The stimulatory effects were completely blocked by specific PPARγ antagonist GW9662, ERK kinase inhibitor PD98059, and CD36 inhibitor sulfosuccinimidyl oleate. Treatment with an siRNA against PPAR gamma completely blocked the stimulation of both NBCe1 and NHE by azPC. Moreover, azPC induced ERK phosphorylation in rat and human kidney cortex tissues, which were completely suppressed by GW9662 and PD98059 treatments. These results suggest that azPC stimulates renal PT sodium-coupled bicarbonate transport via a CD36/PPARγ/mitogen-activated protein/ERK kinase/ERK pathway. We conclude that the stimulatory effects of azPC on PT transport may be partially involved in volume expansion.

Characterization of the Structural Diversity and Structure-Specific Behavior of Oxidized Phospholipids by LC-MS/MS

Polyunsaturated fatty acids (PUFAs), esterified to phospholipids, are susceptible to oxidation. They form oxidized phospholipids (OxPLs) by oxygenases or reactive oxygen species (ROS), or both. These OxPLs are associated with various diseases, such as atherosclerosis, pulmonary injuries, neurodegenerative diseases, cancer, and diabetes. Since many types of OxPLs seem to be generated in vivo, precise determination of their structural diversity is required to understand their potential structure-specific functions. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is a powerful method to quantitatively measure the structural diversity of OxPLs present in biological samples. This review outlines recent advances in analytical methods for OxPLs and their physiological relevance in health and diseases.

Cellular Crosstalk between Endothelial and Smooth Muscle Cells in Vascular Wall Remodeling

Pathological vascular wall remodeling refers to the structural and functional changes of the vessel wall that occur in response to injury that eventually leads to cardiovascular disease (CVD). Vessel wall are composed of two major primary cells types, endothelial cells (EC) and vascular smooth muscle cells (VSMCs). The physiological communications between these two cell types (EC–VSMCs) are crucial in the development of the vasculature and in the homeostasis of mature vessels. Moreover, aberrant EC–VSMCs communication has been associated to the promotor of various disease states including vascular wall remodeling. Paracrine regulations by bioactive molecules, communication via direct contact (junctions) or information transfer via extracellular vesicles or extracellular matrix are main crosstalk mechanisms. Identification of the nature of this EC–VSMCs crosstalk may offer strategies to develop new insights for prevention and treatment of disease that curse with vascular remodeling. Here, we will review the molecular mechanisms underlying the interplay between EC and VSMCs. Additionally, we highlight the potential applicable methodologies of the co-culture systems to identify cellular and molecular mechanisms involved in pathological vascular wall remodeling, opening questions about the future research directions.

Structural, functional, and mechanistic insights uncover the fundamental role of orphan connexin-62 in platelets

Connexins oligomerise to form hexameric hemichannels in the plasma membrane that can further dock together on adjacent cells to form gap junctions and facilitate intercellular trafficking of molecules. In this study, we report the expression and function of an orphan connexin, connexin-62 (Cx62), in human and mouse (Cx57, mouse homolog) platelets. A novel mimetic peptide (62Gap27) was developed to target the second extracellular loop of Cx62, and 3-dimensional structural models predicted its interference with gap junction and hemichannel function. The ability of 62Gap27 to regulate both gap junction and hemichannel-mediated intercellular communication was observed using fluorescence recovery after photobleaching analysis and flow cytometry. Cx62 inhibition by 62Gap27 suppressed a range of agonist-stimulated platelet functions and impaired thrombosis and hemostasis. This was associated with elevated protein kinase A-dependent signaling in a cyclic adenosine monophosphate-independent manner and was not observed in Cx57-deficient mouse platelets (in which the selectivity of 62Gap27 for this connexin was also confirmed). Notably, Cx62 hemichannels were observed to function independently of Cx37 and Cx40 hemichannels. Together, our data reveal a fundamental role for a hitherto uncharacterized connexin in regulating the function of circulating cells.

Direct Targets and Subsequent Pathways for Molecular Hydrogen to Exert Multiple Functions: Focusing on Interventions in Radical Reactions

Molecular hydrogen (H₂) was long regarded as non-functional in mammalian cells. We overturned the concept by demonstrating that H₂ exhibits antioxidant effects and protects cells against oxidative stress. Subsequently, it has been revealed that H₂ has multiple functions in addition to antioxidant effects, including antiinflammatory, anti-allergic functions, and as cell death and autophagy regulation. Additionally, H₂ stimulates energy metabolism. As H₂ does not readily react with most biomolecules without a catalyst, it is essential to identify the primary targets with which H₂ reacts or interacts directly. As a first event, H₂ may react directly with strong oxidants, such as hydroxyl radicals (•OH) in vivo. This review addresses the key issues related to this in vivo reaction. •OH may have a physiological role because it triggers a free radical chain reaction and may be involved in the regulation of Ca2+- or mitochondrial ATP-dependent K+-channeling. In the subsequent pathway, H₂ suppressed a free radical chain reaction, leading to decreases in lipid peroxide and its end products. Derived from the peroxides, 4-hydroxy-2-nonenal functions as a mediator that up-regulates multiple functional PGC-1α. As the other direct target in vitro and in vivo, H₂ intervenes in the free radical chain reaction to modify oxidized phospholipids, which may act as an antagonist of Ca2+-channels. The resulting suppression of Ca2+-signaling inactivates multiple functional NFAT and CREB transcription factors, which may explain H₂ multi-functionality. This review also addresses the involvement of NFAT in the beneficial role of H₂ in COVID-19, Alzheimer's disease and advanced cancer. We discuss some unsolved issues of H₂ action on lipopolysaccharide signaling, MAPK and NF-κB pathways and the Nrf2 paradox. Finally, as a novel idea for the direct targeting of H2, this review introduces the possibility that H₂ causes structural changes in proteins via hydrate water changes.

Src Regulation of Cx43 Phosphorylation and Gap Junction Turnover

The gap junction protein Connexin43 (Cx43) is highly regulated by phosphorylation at over a dozen sites by probably at least as many kinases. This Cx43 “kinome” plays an important role in gap junction assembly and turnover. We sought to gain a better understanding of the interrelationship of these phosphorylation events particularly related to src activation and Cx43 turnover. Using state-of-the-art live imaging methods, specific inhibitors and many phosphorylation-status specific antibodies, we found phospho-specific domains in gap junction plaques and show evidence that multiple pathways of disassembly exist and can be regulated at the cellular and subcellular level. We found Src activation promotes formation of connexisomes (internalized gap junctions) in a process involving ERK-mediated phosphorylation of S279/282. Proteasome inhibition dramatically and rapidly restored gap junctions in the presence of Src and led to dramatic changes in the Cx43 phospho-profile including to increased Y247, Y265, S279/282, S365, and S373 phosphorylation. Lysosomal inhibition, on the other hand, nearly eliminated phosphorylation on Y247 and Y265 and reduced S368 and S373 while increasing S279/282 phosphorylation levels. We present a model of gap junction disassembly where multiple modes of disassembly are regulated by phosphorylation and can have differential effects on cellular signaling.

The role of oxidized phospholipids in the development of disease

Oxidized phospholipids (OxPLs), complex mixtures of phospholipid oxidation products generated during normal or pathological processes, are increasingly recognized to show bioactive effects on many cellular signalling pathways. There is a growing body of evidence showing that OxPLs play an important role in many diseases, so it is essential to define the specific role of OxPLs in different diseases for the design of disease therapies. In vastly diverse pathological processes, OxPLs act as pro-inflammatory agents and contribute to the progression of many diseases; in addition, they play a role in anti-inflammatory processes, promoting the dissipation of inflammation and inhibiting the progression of some diseases. In addition to participating in the regulation of inflammatory responses, OxPLs affect the occurrence and development of diseases through other pathways, such as apoptosis promotion. In this review, the different and even opposite effects of different OxPL molecular species are discussed. Furthermore, the specific effects of OxPLs in various diseases, as well as the receptor and cellular mechanisms involved, are summarized.

Heterocellular Contact Can Dictate Arterial Function

RATIONALE

Resistance arteries and conduit arteries rely on different relative contributions of endothelial-derived hyperpolarization versus nitric oxide to achieve dilatory heterocellular signaling. Anatomically, resistance arteries use myoendothelial junctions (MEJs), endothelial cell projections that make contact with smooth muscle cells. Conduit arteries have very few to no MEJs.

OBJECTIVE

Determine if the presence of MEJs in conduit arteries can alter heterocellular signaling.

METHODS AND RESULTS

We previously demonstrated that PAI-1 (plasminogen activator inhibitor-1) can regulate formation of MEJs. Thus, we applied pluronic gel containing PAI-1 directly to conduit arteries (carotid arteries) to determine if this could induce formation of MEJs. We found a significant increase in endothelial cell projections resembling MEJs that correlated with increased biocytin dye transfer from endothelial cells to smooth muscle cells. Next, we used pressure myography to investigate whether these structural changes were accompanied by a functional change in vasodilatory signaling. Interestingly, PAI-1-treated carotids underwent a switch from a conduit to resistance artery vasodilatory profile via diminished nitric oxide signaling and increased endothelial-derived hyperpolarization signaling in response to the endothelium-dependent agonists acetylcholine and NS309. After PAI-1 application, we also found a significant increase in carotid expression of endothelial alpha globin, a protein predominantly expressed in resistance arteries. Carotids from mice with PAI-1, but lacking alpha globin (Hba1^-/-), demonstrated that l-nitro-arginine methyl ester, an inhibitor of nitric oxide signaling, was able to prevent arterial relaxation.

CONCLUSIONS

The presence or absence of MEJs is an important determinant for influencing heterocellular communication in the arterial wall. In particular, alpha globin expression, induced within newly formed endothelial cell projections, may influence the balance between endothelial-derived hyperpolarization and nitric oxide-mediated vasodilation.

Lipoprotein(a) the Insurgent: A New Insight into the Structure, Function, Metabolism, Pathogenicity, and Medications Affecting Lipoprotein(a) Molecule

Lipoprotein(a) [Lp(a)], aka "Lp little a", was discovered in the 1960s in the lab of the Norwegian physician Kåre Berg. Since then, we have greatly improved our knowledge of lipids and cardiovascular disease (CVD). Lp(a) is an enigmatic class of lipoprotein that is exclusively formed in the liver and comprises two main components, a single copy of apolipoprotein (apo) B-100 (apo-B100) tethered to a single copy of a protein denoted as apolipoprotein(a) apo(a). Plasma levels of Lp(a) increase soon after birth to a steady concentration within a few months of life. In adults, Lp(a) levels range widely from <2 to 2500 mg/L. Evidence that elevated Lp(a) levels >300 mg/L contribute to CVD is significant. The improvement of isoform-independent assays, together with the insight from epidemiologic studies, meta-analyses, genome-wide association studies, and Mendelian randomization studies, has established Lp(a) as the single most common independent genetically inherited causal risk factor for CVD. This breakthrough elevated Lp(a) from a biomarker of atherosclerotic risk to a target of therapy. With the emergence of promising second-generation antisense therapy, we hope that we can answer the question of whether Lp(a) is ready for prime-time clinic use. In this review, we present an update on the metabolism, pathophysiology, and current/future medical interventions for high levels of Lp(a).

Morphological and pharmacological characterization of the porcine popliteal artery: A novel model for study of lower limb arterial disease

OBJECTIVE

This study was undertaken to characterize structural and pharmacological properties of the pig popliteal artery in order to develop a novel system for the examination of lower limb blood flow regulation in a variety of cardiovascular pathologies, such as diabetes-induced peripheral artery disease.

METHODS

Popliteal arteries were isolated from streptozocin-induced diabetic pigs or age-matched saline-injected control pigs for morphological study using transmission electron microscopy and for examination of vasoreactivity to pharmacological agents using wire myography.

RESULTS

Transmission electron microscopy of the porcine popliteal artery wall revealed the presence of endothelial cell-smooth muscle cell interactions (myoendothelial junctions) and smooth muscle cell-smooth muscle cell interactions, for which we have coined the term "myo-myo junctions." These myo-myo junctions were shown to feature plaques indicative of connexin expression. Further, the pig popliteal artery was highly responsive to a variety of vasoconstrictors including norepinephrine, phenylephrine, and U46619, and vasodilators including acetylcholine, adenosine 5'-[β-thio] diphosphate, and bradykinin. Finally, 2 weeks after streptozocin-induced diabetes, the normalized vasoconstriction of the pig popliteal artery to norepinephrine was unaltered compared to control.

CONCLUSIONS

The pig popliteal artery displays structural and pharmacological properties that might prove useful in future studies of diabetes-associated peripheral artery disease and other lower limb cardiovascular diseases.

The Functional Implications of Endothelial Gap Junctions and Cellular Mechanics in Vascular Angiogenesis

Angiogenesis—the sprouting and growth of new blood vessels from the existing vasculature—is an important contributor to tumor development, since it facilitates the supply of oxygen and nutrients to cancer cells. Endothelial cells are critically affected during the angiogenic process as their proliferation, motility, and morphology are modulated by pro-angiogenic and environmental factors associated with tumor tissues and cancer cells. Recent in vivo and in vitro studies have revealed that the gap junctions of endothelial cells also participate in the promotion of angiogenesis. Pro-angiogenic factors modulate gap junction function and connexin expression in endothelial cells, whereas endothelial connexins are involved in angiogenic tube formation and in the cell migration of endothelial cells. Several mechanisms, including gap junction function-dependent or -independent pathways, have been proposed. In particular, connexins might have the potential to regulate cell mechanics such as cell morphology, cell migration, and cellular stiffness that are dynamically changed during the angiogenic processes. Here, we review the implication for endothelial gap junctions and cellular mechanics in vascular angiogenesis.

Oxidized lipids: not just another brick in the wall 1

Over the past decade, there has been intense investigation in trying to understand the pathological role that oxidized phospholipids play in cardiovascular disease. Phospholipids are targets for oxidation, particularly during conditions of excess free radical generation. Once oxidized, they acquire novel roles uncharacteristic of their precursors. Oxidized phosphatidylcholines have an important role in multiple physiological and pathophysiological conditions including atherosclerosis, neurodegenerative diseases, lung disease, inflammation, and chronic alcohol consumption. Circulating oxidized phosphatidylcholine may also serve as a clinical biomarker. The focus of this review, therefore, will be to summarize existing evidence that oxidized phosphatidylcholine molecules play an important role in cardiovascular pathology.

Connexins and Pannexins in Vascular Function and Disease

Connexins (Cxs) and pannexins (Panxs) are ubiquitous membrane channel forming proteins that are critically involved in many aspects of vascular physiology and pathology. The permeation of ions and small metabolites through Panx channels, Cx hemichannels and gap junction channels confers a crucial role to these proteins in intercellular communication and in maintaining tissue homeostasis. This review provides an overview of current knowledge with respect to the pathophysiological role of these channels in large arteries, the microcirculation, veins, the lymphatic system and platelet function. The essential nature of these membrane proteins in vascular homeostasis is further emphasized by the pathologies that are linked to mutations and polymorphisms in Cx and Panx genes.

Connexins: Synthesis, Post-Translational Modifications, and Trafficking in Health and Disease

Connexins are tetraspan transmembrane proteins that form gap junctions and facilitate direct intercellular communication, a critical feature for the development, function, and homeostasis of tissues and organs. In addition, a growing number of gap junction-independent functions are being ascribed to these proteins. The connexin gene family is under extensive regulation at the transcriptional and post-transcriptional level, and undergoes numerous modifications at the protein level, including phosphorylation, which ultimately affects their trafficking, stability, and function. Here, we summarize these key regulatory events, with emphasis on how these affect connexin multifunctionality in health and disease.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

Pleiotropic effects of oxidized phospholipids

Oxidized phospholipids (OxPLs) are increasingly recognized to play a role in a variety of normal and pathological states. OxPLs were implicated in regulation of inflammation, thrombosis, angiogenesis, endothelial barrier function, immune tolerance and other important processes. Rapidly accumulating evidence suggests that OxPLs are biomarkers of atherosclerosis and other pathologies. In addition, successful application of experimental drugs based on structural scaffold of OxPLs in animal models of inflammation was recently reported. This review briefly summarizes current knowledge on generation, methods of quantification and biological activities of OxPLs. Furthermore, receptor and cellular mechanisms of these effects are discussed. The goal of the review is to give a broad overview of this class of lipid mediators inducing pleiotropic biological effects.

Lysophosphatidic acid-induced vascular neointimal formation in mouse carotid arteries is mediated by the matricellular protein CCN1/Cyr61

Vascular smooth muscle cell (SMC) migration is an essential step involved in neointimal formation in restenosis and atherosclerosis. Lysophosphatidic acid (LPA) is a bioactive component of oxidized low-density lipoprotein and is produced by activated platelets, implying that LPA influences vascular remodeling. Our previous study revealed that matricellular protein CCN1, a prominent extracellular matrix (ECM) protein, mediates LPA-induced SMC migration in vitro. Here we examined the role of CCN1 in LPA-induced neointimal formation. By using LPA infusion of carotid artery in a mouse model, we demonstrated that LPA highly induced CCN1 expression (approximately six- to sevenfold) in neointimal lesions. Downregulation of CCN1 expression with the specific CCN1 siRNA in carotid arteries blocked LPA-induced neointimal formation, indicating that CCN1 is essential in LPA-induced neointimal formation. We then used LPA receptor knockout (LPA₁-/-, LPA₂-/-, and LPA₃-/-) mice to examine LPA receptor function in CCN1 expression in vivo and in LPA-induced neointimal formation. Our data reveal that LPA₁ deficiency, but not LPA₂ or LPA₃ deficiency, prevents LPA-induced CCN1 expression in vivo in mouse carotid arteries. We also observed that LPA₁ deficiency blunted LPA infusion-induced neointimal formation, indicating that LPA₁ is the major mediator for LPA-induced vascular remodeling. Our in vivo model of LPA-induced neointimal formation established a key role of the ECM protein CCN1 in mediating LPA-induced neointimal formation. Our data support the notion that the LPA₁-CCN1 axis may be the central control for SMC migration and vascular remodeling. CCN1 may serve as an important vascular disease marker and potential target for vascular therapeutic intervention.

Identification of Connexin43 Phosphorylation and S-Nitrosylation in Cultured Primary Vascular Cells

All connexins (Cx) proteins contain both highly ordered domains (i.e., 4 transmembrane domains) and primarily unstructured regions (i.e., n- and c-terminal domains). The c-terminal domains vary in length and amino acid composition from the shortest on Cx26 to the longest on Cx43. With the exception of Cx26, the c-terminal domains contain multiple sites for posttranslational modification (PTM) including serines (S), threonines (T), and tyrosines (Y) for phosphorylation or cysteines (C) for S-nitrosylation. These PTMs are critical for regulating cellular localization, protein-protein interactions, and channel functionality. There are several biochemical techniques that allow for the identification of these PTM including Western blotting and the "Biotin Switch" assay for nitrosylation. Quantitative analysis of Western blots can be achieved through use of secondary antibodies in the near infrared scale and high-resolution scanning on a fluorescent scanner.

Connexin 43 is an emerging therapeutic target in ischemia/reperfusion injury, cardioprotection and neuroprotection

Connexins are widely distributed proteins in the body that are crucially important for heart and brain functions. Six connexin subunits form a connexon or hemichannel in the plasma membrane. Interactions between two hemichannels in a head-to-head arrangement result in the formation of a gap junction channel. Gap junctions are necessary to coordinate cell function by passing electrical current flow between heart and nerve cells or by allowing exchange of chemical signals and energy substrates. Apart from its localization at the sarcolemma of cardiomyocytes and brain cells, connexins are also found in the mitochondria where they are involved in the regulation of mitochondrial matrix ion fluxes and respiration. Connexin expression is affected by age and gender as well as several pathophysiological alterations such as hypertension, hypertrophy, diabetes, hypercholesterolemia, ischemia, post-myocardial infarction remodeling or heart failure, and post-translationally connexins are modified by phosphorylation/de-phosphorylation and nitros(yl)ation which can modulate channel activity. Using knockout/knockin technology as well as pharmacological approaches, one of the connexins, namely connexin 43, has been identified to be important for cardiac and brain ischemia/reperfusion injuries as well as protection from it. Therefore, the current review will focus on the importance of connexin 43 for irreversible injury of heart and brain tissues following ischemia/reperfusion and will highlight the importance of connexin 43 as an emerging therapeutic target in cardio- and neuroprotection.

Differential effects of chlorinated and oxidized phospholipids in vascular tissue: implications for neointima formation

The presence of inflammatory cells and MPO (myeloperoxidase) in the arterial wall after vascular injury could increase neointima formation by modification of phospholipids. The present study investigates how these phospholipids, in particular oxidized and chlorinated species, are altered within injured vessels and how they affect VSMC (vascular smooth muscle cell) remodelling processes. Vascular injury was induced in C57BL/6 mice and high fat-fed ApoE-/- (apolipoprotein E) mice by wire denudation and ligation of the left carotid artery (LCA). Neointimal and medial composition was assessed using immunohistochemistry and ESI-MS. Primary rabbit aortic SMCs (smooth muscle cells) were utilized to examine the effects of modified lipids on VSMC proliferation, viability and migration at a cellular level. Neointimal area, measured as intima-to-media ratio, was significantly larger in wire-injured ApoE-/- mice (3.62±0.49 compared with 0.83±0.25 in C57BL/6 mice, n=3) and there was increased oxidized low-density lipoprotein (oxLDL) infiltration and elevated plasma MPO levels. Relative increases in lysophosphatidylcholines and unsaturated phosphatidylcholines (PCs) were also observed in wire-injured ApoE-/- carotid arteries. Chlorinated lipids had no effect on VSMC proliferation, viability or migration whereas chronic incubation with oxidized phospholipids stimulated proliferation in the presence of fetal calf serum [154.8±14.2% of viable cells at 1 μM PGPC (1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine) compared with control, n=6]. In conclusion, ApoE-/- mice with an inflammatory phenotype develop more neointima in wire-injured arteries and accumulation of oxidized lipids in the vessel wall may propagate this effect.

Recombinant PAI-1 therapy restores myoendothelial junctions and erectile function in PAI-1-deficient mice

Myoendothelial junctions are specialised projections of cell : cell contact through the internal elastic lamina between endothelial cells and vascular smooth muscle cells. These junctions allow for endothelial cells and vascular smooth muscle cells to make direct membrane apposition and are involved in cell : cell communication. In this study, we evaluated for the presence of myoendothelial junctions in murine corporal tissue and used plasminogen activator inhibitor (PAI)-1-deficient mice, which lack myoendothelial junctions, to determine whether myoendothelial junctions affect erectile function. Transmission electron microscopy demonstrated the presence of myoendothelial junctions in the corporal tissue of wild-type mice and confirmed the decreased junction numbers in the tissue of PAI-1(-/-) mice. A potential role for myoendothelial junctions in tumescence was established; in that, PAI-1(-/-) mice demonstrated a significantly longer time to achieve maximal intracavernous pressure. Treatment of PAI-1(-/-) mice with recombinant PAI-1 restored the number of myoendothelial junctions in the corporal tissue and also induced a significant decrease in time to maximal corporal pressures. Myoendothelial junctions were similarly identified in the human corporal tissue. These results suggest a critical role for myoendothelial junctions in erectile pathophysiology and therapies aimed at restoring myoendothelial junction numbers in the corporal tissue may provide a novel therapy for erectile dysfunction.

Lipid peroxidation generates biologically active phospholipids including oxidatively N-modified phospholipids

Peroxidation of membranes and lipoproteins converts "inert" phospholipids into a plethora of oxidatively modified phospholipids (oxPL) that can act as signaling molecules. In this review, we will discuss four major classes of oxPL: mildly oxygenated phospholipids, phospholipids with oxidatively truncated acyl chains, phospholipids with cyclized acyl chains, and phospholipids that have been oxidatively N-modified on their headgroups by reactive lipid species. For each class of oxPL we will review the chemical mechanisms of their formation, the evidence for their formation in biological samples, the biological activities and signaling pathways associated with them, and the catabolic pathways for their elimination. We will end by briefly highlighting some of the critical questions that remain about the role of oxPL in physiology and disease.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Specific Cx43 phosphorylation events regulate gap junction turnover in vivo

Gap junctions, composed of proteins from the connexin gene family, are highly dynamic structures that are regulated by kinase-mediated signaling pathways and interactions with other proteins. Phosphorylation of Connexin43 (Cx43) at different sites controls gap junction assembly, gap junction size and gap junction turnover. Here we present a model describing how Akt, mitogen activated protein kinase (MAPK) and src kinase coordinate to regulate rapid turnover of gap junctions. Specifically, Akt phosphorylates Cx43 at S373 eliminating interaction with zona occludens-1 (ZO-1) allowing gap junctions to enlarge. Then MAPK and src phosphorylate Cx43 to initiate turnover. We integrate published data with new data to test and refine this model. Finally, we propose that differential coordination of kinase activation and Cx43 phosphorylation controls the specific routes of disassembly, e.g., annular junction formation or gap junctions can potentially "unzip" and be internalized/endocytosed into the cell that produced each connexin.

Gap junction and hemichannel-independent actions of connexins on cell and tissue functions--an update

Connexins, a family of transmembrane proteins, are components of both gap junction channels and hemichannels, which mediate the exchange of ions and small molecules between adjacent cells, and between the inside and outside of the cell, respectively. Substantial advancements have been made in the comprehension of the role of gap junctions and hemichannels in coordinating cellular events. In recent years, a plethora of studies demonstrate a role of connexin proteins in the regulation of tissue homeostasis that occurs independently of their channel activities. This is shown in the context of cell growth, adhesion, migration, apoptosis, and signaling. The major mechanisms of these channel-independent activities still remain to be discovered. In this review, we provide an updated overview on the current knowledge of gap junction- and hemichannel-independent functions of connexins, in particular, their effects on tumorigenesis, neurogenesis and disease development.

Inhibition of oxidized-phospholipid-induced vascular smooth muscle cell proliferation by resveratrol is associated with reducing Cx43 phosphorylation

Abnormal proliferation of vascular smooth muscle cells (VSMCs) is an important factor during the progression of atherosclerosis. In this study, we investigated the effects of resveratrol on atherosclerosis-associated proliferation of VSMCs. We utilized an oxidized phospholipid, 1-palmitoyl-2-oxovaleroyl-sn-glycero-3-phosphorylcholine (POVPC) to induce abnormal proliferation of VSMCs. Our results showed the treatments with resveratrol dose-dependently abolished POVPC-induced VSMC proliferation, as evidenced by the decreased [(3)H]thymidine incorporated into VSMCs and reduced percentage of 5-ethynyl-2'-deoxyuridine (EdU)-positive VSMCs. Cell cycle analysis demonstrated that resveratrol inhibited POVPC-induced increase in the S phase cell population and DNA synthesis. Our study further indicated that POVPC-induced VSMC proliferation was associated with a significant increase in the phosphorylation of Cx43, which was a consequence of activation of MAPK signaling. Interestingly, treatment with resveratrol abolished POVPC-induced phosphorylation of Cx43 as a result of inhibiting activation of Src, MEK, and ERK1/2. Our results provided a novel mechanism by which resveratrol may contribute to cardiovascular protection.

Vascular smooth muscle cells in cerebral aneurysm pathogenesis

Vascular smooth muscle cells (SMC) maintain significant plasticity. Following environmental stimulation, SMC can alter their phenotype from one primarily concerned with contraction to a pro-inflammatory and matrix remodeling phenotype. This is a critical process behind peripheral vascular disease and atherosclerosis, a key element of cerebral aneurysm pathology. Evolving evidence demonstrates that SMCs and phenotypic modulation play a significant role in cerebral aneurysm formation and rupture. Pharmacological alteration of smooth muscle cell function and phenotypic modulation could provide a promising medical therapy to inhibit cerebral aneurysm progression. This study reviews vascular SMC function and its contribution to cerebral aneurysm pathophysiology.

Saturated fatty acid palmitate aggravates neointima formation by promoting smooth muscle phenotypic modulation

OBJECTIVE

Obesity is a major risk factor of atherosclerotic cardiovascular disease. Circulating free fatty acid levels are known to be elevated in obese individuals and, along with dietary saturated fatty acids, are known to associate with cardiovascular events. However, little is known about the molecular mechanisms by which free fatty acids are linked to cardiovascular disease.

APPROACH AND RESULTS

We found that administration of palmitate, a major saturated free fatty acid, to mice markedly aggravated neointima formation induced by carotid artery ligation and that the neointima primarily consisted of phenotypically modulated smooth muscle cells (SMCs). In cultured SMCs, palmitate-induced phenotypic modulation was characterized by downregulation of SMC differentiation markers, such as SM α-actin and SM-myosin heavy chain, and upregulation of mediators involved in inflammation and remodeling of the vessel wall, such as platelet-derived growth factor B and matrix metalloproteinases. We also found that palmitate induced the expression of proinflammatory genes via a novel toll-like receptor 4/myeloid differentiation primary response 88/nuclear factor-κB/NADPH oxidase 1/reactive oxygen species signaling pathway: nuclear factor-κB was activated by palmitate via toll-like receptor 4 and its adapter, MyD88, and once active, it transactivated Nox1, encoding NADPH oxidase 1, a major reactive oxygen species generator in SMCs. Pharmacological inhibition and small interfering RNA-mediated knockdown of the components of this signaling pathway mitigated the palmitate-induced upregulation of proinflammatory genes. More importantly, Myd88 knockout mice were resistant to palmitate-induced exacerbation of neointima formation.

CONCLUSIONS

Palmitate seems to promote neointima formation by inducing inflammatory phenotypes in SMCs.

Regulation of connexin- and pannexin-based channels by post-translational modifications

Connexin (Cx) and pannexin (Panx) proteins form large conductance channels, which function as regulators of communication between neighbouring cells via gap junctions and/or hemichannels. Intercellular communication is essential to coordinate cellular responses in tissues and organs, thereby fulfilling an essential role in the spreading of signalling, survival and death processes. The functional properties of gap junctions and hemichannels are modulated by different physiological and pathophysiological stimuli. At the molecular level, Cxs and Panxs function as multi-protein channel complexes, regulating their channel localisation and activity. In addition to this, gap junctional channels and hemichannels are modulated by different post-translational modifications (PTMs), including phosphorylation, glycosylation, proteolysis, N-acetylation, S-nitrosylation, ubiquitination, lipidation, hydroxylation, methylation and deamidation. These PTMs influence almost all aspects of communicating junctional channels in normal cell biology and pathophysiology. In this review, we will provide a systematic overview of PTMs of communicating junction proteins and discuss their effects on Cx and Panx-channel activity and localisation.

Role of phospholipid oxidation products in atherosclerosis

There is increasing clinical evidence that phospholipid oxidation products (Ox-PL) play a role in atherosclerosis. This review focuses on the mechanisms by which Ox-PL interact with endothelial cells, monocyte/macrophages, platelets, smooth muscle cells, and HDL to promote atherogenesis. In the past few years major progress has been made in identifying these mechanisms. It has been recognized that Ox-PL promote phenotypic changes in these cell types that have long-term consequences for the vessel wall. Individual Ox-PL responsible for specific cellular effects have been identified. A model of the configuration of bioactive truncated Ox-PL within membranes has been developed that demonstrates that the oxidized fatty acid moiety protrudes into the aqueous phase, rendering it accessible for receptor recognition. Receptors and signaling pathways for individual Ox-PL species are now determined and receptor independent signaling pathways identified. The effects of Ox-PL are mediated both by gene regulation and transcription independent processes. It has now become apparent that Ox-PL affects multiple genes and pathways, some of which are proatherogenic and some are protective. However, at concentrations that are likely present in the vessel wall in atherosclerotic lesions, the effects promote atherogenesis. There have also been new insights on enzymes that metabolize Ox-PL and the significance of these enzymes for atherosclerosis. With the knowledge we now have of the regulation and effects of Ox-PL in different vascular cell types, it should be possible to design experiments to test the role of specific Ox-PL on the development of atherosclerosis.

Characterization of the thoracodorsal artery: morphology and reactivity

OBJECTIVES

In this paper, we describe the histological and contractile properties of the thoracodorsal artery (TDA), which indirectly feeds the spinotrapezius muscle.

METHODS

We used immunolabelling techniques to histologically characterize the TDA while the contractile properties were assessed using pressure arteriography.

RESULTS

Our results demonstrate that the TDA is composed of approximately one to two layers of smooth muscle cells, is highly innervated with adrenergic nerves, and develops spontaneous tone at intraluminal pressures above 80 mmHg. The reactivity of the TDA in response to various contractile agonists such as phenylephrine, noradrenaline, angiotensin II, serotonin, endothelin 1, and ATP, as well as vasodilators, shows that the TDA exhibits a remarkably comparable reactivity to what has been observed in mesenteric arteries. We further studied the different components of the TDA response to acetylcholine, and found that the TDA was sensitive to TRAM 34, a blocker of the intermediate conductance potassium channel, which is highly suggestive of an endothelium-dependent hyperpolarization.

CONCLUSIONS

We conclude that the TDA exhibits comparable characteristics to other current vascular models, with the additional advantage of being easily manipulated for molecular and ex vivo vasoreactivity studies.

Posttranslational modifications in connexins and pannexins

Posttranslational modification is a common cellular process that is used by cells to ensure a particular protein function. This can happen in a variety of ways, e.g., from the addition of phosphates or sugar residues to a particular amino acid, ensuring proper protein life cycle and function. In this review, we assess the evidence for ubiquitination, glycosylation, phosphorylation, S-nitrosylation as well as other modifications in connexins and pannexin proteins. Based on the literature, we find that posttranslational modifications are an important component of connexin and pannexin regulation.

Expression of pannexin isoforms in the systemic murine arterial network

AIMS

Pannexins (Panx) form ATP release channels and it has been proposed that they play an important role in the regulation of vascular tone. However, distribution of Panx across the arterial vasculature is not documented.

METHODS

We tested antibodies against Panx1, Panx2 and Panx3 on human embryonic kidney cells (which do not endogenously express Panx proteins) transfected with plasmids encoding each Panx isoform and Panx1(-/-) mice. Each of the Panx antibodies was found to be specific and was tested on isolated arteries using immunocytochemistry.

RESULTS

We demonstrated that Panx1 is the primary isoform detected in the arterial network. In large arteries, Panx1 is primarily in endothelial cells, whereas in small arteries and arterioles it localizes primarily to the smooth muscle cells. Panx1 was the predominant isoform expressed in coronary arteries, except in arteries less than 100 µm where Panx3 became detectable. Only Panx3 was expressed in the juxtaglomerular apparatus and cortical arterioles. The pulmonary artery and alveoli had expression of all 3 Panx isoforms. No Panx isoforms were detected at the myoendothelial junctions.

CONCLUSION

We conclude that the specific localized expression of Panx channels throughout the vasculature points towards an important role for these channels in regulating the release of ATP throughout the arterial network.

MAPK phosphorylation of connexin 43 promotes binding of cyclin E and smooth muscle cell proliferation

RATIONALE

Dedifferentiation of vascular smooth muscle cells (VSMC) leading to a proliferative cell phenotype significantly contributes to the development of atherosclerosis. Mitogen-activated protein kinase (MAPK) phosphorylation of proteins including connexin 43 (Cx43) has been associated with VSMC proliferation in atherosclerosis.

OBJECTIVE

To investigate whether MAPK phosphorylation of Cx43 is directly involved in VSMC proliferation.

METHODS AND RESULTS

We show in vivo that MAPK-phosphorylated Cx43 forms complexes with the cell cycle control proteins cyclin E and cyclin-dependent kinase 2 (CDK2) in carotids of apolipoprotein-E receptor null (ApoE(-/-)) mice and in C57Bl/6 mice treated with platelet-derived growth factor-BB (PDGF). We tested the involvement of Cx43 MAPK phosphorylation in vitro using constructs for full-length Cx43 (Cx43) or the Cx43 C-terminus (Cx43(CT)) and produced null phosphorylation Ser>Ala (Cx43(MK4A)/Cx43(CTMK4A)) and phospho-mimetic Ser>Asp (Cx43(MK4D)/Cx43(CTMK4D)) mutations. Coimmunoprecipitation studies in primary VSMC isolated from Cx43 wild-type (Cx43(+/+)) and Cx43 null (Cx43(-/-)) mice and analytic size exclusion studies of purified proteins identify that interactions between cyclin E and Cx43 requires Cx43 MAPK phosphorylation. We further demonstrate that Cx43 MAPK phosphorylation is required for PDGF-mediated VSMC proliferation. Finally, using a novel knock-in mouse containing Cx43-MK4A mutation, we show in vivo that interactions between Cx43 and cyclin E are lost and VSMC proliferation does not occur after treatment of carotids with PDGF and that neointima formation is significantly reduced in carotids after injury.

CONCLUSIONS

We identify MAPK-phosphorylated Cx43 as a novel interacting partner of cyclin E in VSMC and show that this interaction is critical for VSMC proliferation. This novel interaction may be important in the development of atherosclerotic lesions.

Biophysics of lipid bilayers containing oxidatively modified phospholipids: insights from fluorescence and EPR experiments and from MD simulations

This review focuses on the influence of oxidized phosphatidylcholines (oxPCs) on the biophysical properties of model membranes and is limited to fluorescence, EPR, and MD studies. OxPCs are divided into two classes: A) hydroxy- or hydroperoxy-dieonyl phospatidylcholines, B) phospatidylcholines with oxidized and truncated chains with either aldehyde or carboxylic group. It was shown that the presence of the investigated oxPCs in phospholipid model membranes may have the following consequences: 1) decrease of the lipid order, 2) lowering of phase transition temperatures, 3) lateral expansion and thinning of the bilayer, 4) alterations of bilayer hydration profiles, 5) increased lipid mobility, 6) augmented flip-flop, 7) influence on the lateral phase organisation, and 8) promotion of water defects and, under extreme conditions (i.e. high concentrations of class B oxPCs), disintegration of the bilayer. The effects of class A oxPCs appear to be more moderate than those observed or predicted for class B. Many of the abovementioned findings are related to the ability of the oxidized chains of certain oxPCs to reorient toward the water phase. Some of the effects appear to be moderated by the presence of cholesterol. Although those biophysical alternations are found at oxPC concentrations higher than the total oxPC concentrations found under physiological conditions, certain organelles may reach such elevated oxPC concentrations locally. It is a challenge for the future to correlate the biophysics of oxidized phospholipids to metabolic studies in order to define the significance of the findings presented herein for pathophysiology. This article is part of a Special Issue entitled: Oxidized phospholipids-their properties and interactions with proteins.

Connexins in atherosclerosis

Atherosclerosis, a chronic inflammatory disease of the vessel wall, involves multiple cell types of different origins, and complex interactions and signaling pathways between them. Autocrine and paracrine communication pathways provided by cytokines, chemokines, growth factors and lipid mediators are central to atherogenesis. However, it is becoming increasingly recognized that a more direct communication through both hemichannels and gap junction channels formed by connexins also plays an important role in atherosclerosis development. Three main connexins are expressed in cells involved in atherosclerosis: Cx37, Cx40 and Cx43. Cx37 is found in endothelial cells, monocytes/macrophages and platelets, Cx40 is predominantly an endothelial connexin, and Cx43 is found in a large variety of cells such as smooth muscle cells, resident and circulating leukocytes (neutrophils, dendritic cells, lymphocytes, activated macrophages, mast cells) and some endothelial cells. Here, we will systematically review the expression and function of connexins in cells and processes underlying atherosclerosis. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.