Human macrophages synthesize type VIII collagen in vitro and in the atherosclerotic plaque

Collagen VIII in vascular diseases

Collagens have dual functions in the extracellular matrix (ECM), acting as both structural components and signaling molecules in matricellular communication. Although collagen molecules share a common triple helix motif, the supramolecular organization helps classify them into nearly 30 different types of collagens. Collagen type VIII is a non-fibrillar, short-chain, network-forming collagen that is expressed throughout the vasculature. Collagen VIII expression is aberrant in cardiovascular, lung, and renal disease, as well as in several different types of cancer. It plays active roles in angiogenesis, vessel injury repair, maintenance of arterial compliance, atherosclerotic plaque formation and stability modulation, fibrosis, and ECM remodeling. This review presents an overview of the characteristics of collagen VIII in vascular-related disorders, from clinical significance to laboratory studies, with a major focus on highlighting the signaling properties of collagen VIII in the vascular ECM. The expression patterns of collagen VIII in human diseases and experimental animal models highlight the protein's important yet underexplored functions. A deeper understanding of its mechanisms and downstream signaling pathways may pave the way for translational and tissue engineering applications of collagen VIII.

The interplay of collagen, macrophages, and microcalcification in atherosclerotic plaque cap rupture mechanics

The rupture of an atherosclerotic plaque cap overlying a lipid pool and/or necrotic core can lead to thrombotic cardiovascular events. In essence, the rupture of the plaque cap is a mechanical event, which occurs when the local stress exceeds the local tissue strength. However, due to inter- and intra-cap heterogeneity, the resulting ultimate cap strength varies, causing proper assessment of the plaque at risk of rupture to be lacking. Important players involved in tissue strength include the load-bearing collagenous matrix, macrophages, as major promoters of extracellular matrix degradation, and microcalcifications, deposits that can exacerbate local stress, increasing tissue propensity for rupture. This review summarizes the role of these components individually in tissue mechanics, along with the interplay between them. We argue that to be able to improve risk assessment, a better understanding of the effect of these individual components, as well as their reciprocal relationships on cap mechanics, is required. Finally, we discuss potential future steps, including a holistic multidisciplinary approach, multifactorial 3D in vitro model systems, and advancements in imaging techniques. The obtained knowledge will ultimately serve as input to help diagnose, prevent, and treat atherosclerotic cap rupture.

Immunomodulation of Skin Repair: Cell-Based Therapeutic Strategies for Skin Replacement (A Comprehensive Review)

The immune system has a crucial role in skin wound healing and the application of specific cell-laden immunomodulating biomaterials emerged as a possible treatment option to drive skin tissue regeneration. Cell-laden tissue-engineered skin substitutes have the ability to activate immune pathways, even in the absence of other immune-stimulating signals. In particular, mesenchymal stem cells with their immunomodulatory properties can create a specific immune microenvironment to reduce inflammation, scarring, and support skin regeneration. This review presents an overview of current wound care techniques including skin tissue engineering and biomaterials as a novel and promising approach. We highlight the plasticity and different roles of immune cells, in particular macrophages during various stages of skin wound healing. These aspects are pivotal to promote the regeneration of nonhealing wounds such as ulcers in diabetic patients. We believe that a better understanding of the intrinsic immunomodulatory features of stem cells in implantable skin substitutes will lead to new translational opportunities. This, in turn, will improve skin tissue engineering and regenerative medicine applications.

The Roles of Macrophages in Heart Regeneration and Repair After Injury

Although great advances have been made, the problem of irreversible myocardium loss due to the limited regeneration capacity of cardiomyocytes has not been fully solved. The morbidity and mortality of heart disease still remain high. There are many therapeutic strategies for treating heart disease, while low efficacy and high cost remain challenging. Abundant evidence has shown that both acute and chronic inflammations play a crucial role in heart regeneration and repair following injury. Macrophages, a primary component of inflammation, have attracted much attention in cardiac research in recent decades. The detailed mechanisms of the roles of macrophages in heart regeneration and repair are not completely understood, in part because of their complex subsets, various functions, and intercellular communications. The purpose of this review is to summarize the progress made in the understanding of macrophages, including recent reports on macrophage differentiation, polarization and function, and involvement in heart regeneration and repair. Also, we discuss progress in treatments, which may suggest directions for future research.

GM-CSF Calibrates Macrophage Defense and Wound Healing Programs during Intestinal Infection and Inflammation

Macrophages play a central role in intestinal immunity, but inappropriate macrophage activation is associated with inflammatory bowel disease (IBD). Here, we identify granulocyte-macrophage colony stimulating factor (GM-CSF) as a critical regulator of intestinal macrophage activation in patients with IBD and mice with dextran sodium sulfate (DSS)-induced colitis. We find that GM-CSF drives the maturation and polarization of inflammatory intestinal macrophages, promoting anti-microbial functions while suppressing wound-healing transcriptional programs. Group 3 innate lymphoid cells (ILC3s) are a major source of GM-CSF in intestinal inflammation, with a strong positive correlation observed between ILC or CSF2 transcripts and M1 macrophage signatures in IBD mucosal biopsies. Furthermore, GM-CSF-dependent macrophage polarization results in a positive feedback loop that augmented ILC3 activation and type 17 immunity. Together, our data reveal an important role for GM-CSF-mediated ILC-macrophage crosstalk in calibrating intestinal macrophage phenotype to enhance anti-bacterial responses, while inhibiting pro-repair functions associated with fibrosis and stricturing, with important clinical implications.

Metabolism in the Tumor Microenvironment

Experiments in culture systems where one cell type is provided with abundant nutrients and oxygen have been used to inform much of our understanding of cancer metabolism. However, many differences have been observed between the metabolism of tumors and the metabolism of cancer cells grown in monoculture. These differences reflect, at least in part, the presence of nonmalignant cells in the tumor microenvironment and the interactions between those cells and cancer cells. However, less is known about how the metabolism of various tumor stromal cell types differs from that of cancer cells, and how this difference might inform therapeutic targeting of metabolic pathways. Emerging data have identified both cooperative and competitive relationships between different cell types in a tumor, and this review examines how four abundant stromal cell types in the tumor microenvironment, fibroblasts, T cells, macrophages, and endothelial cells, contribute to the metabolism of tumors.

Gene Expression Profiling of the Extracellular Matrix Signature in Macrophages of Different Activation Status: Relevance for Skin Wound Healing

The extracellular matrix (ECM) provides structural support for tissue architecture and is a major effector of cell behavior during skin repair and inflammation. Macrophages are involved in all stages of skin repair but only limited knowledge exists about macrophage-specific expression and regulation of ECM components. In this study, we used transcriptome profiling and bioinformatic analysis to define the unique expression of ECM-associated genes in cultured macrophages. Characterization of the matrisome revealed that most genes were constitutively expressed and that several genes were uniquely regulated upon interferon gamma (IFNγ) and dexamethasone stimulation. Among those core matrisome and matrisome-associated components transforming growth factor beta (TGFβ)-induced, matrix metalloproteinase 9 (MMP9), elastin microfibril interfacer (EMILIN)-1, netrin-1 and gliomedin were also present within the wound bed at time points that are characterized by profound macrophage infiltration. Hence, macrophages are a source of ECM components in vitro as well as during skin wound healing, and identification of these matrisome components is a first step to understand the role and therapeutic value of ECM components in macrophages and during wound healing.

The intestinal tissue homeostasis - the role of extracellular matrix remodeling in inflammatory bowel disease

Introduction: Extracellular matrix (ECM) remodeling of the intestinal tissue is important in inflammatory bowel disease (IBD) due to the extensive mucosal remodeling. There are still gaps in our knowledge as to how ECM remodeling is related to intestinal epithelium homeostasis and healing of the intestinal mucosa.Areas covered: The aim of this review is to highlight the importance of the ECM in relation to the pathogenesis of IBD, while addressing basement membrane and interstitial matrix remodeling, and the processes of wound healing of the intestinal tissue in IBD.Expert opinion: In IBD, basement membrane remodeling may reflect the integrity of the intestinal epithelial-cell homeostasis. The interstitial matrix remodeling is associated with deep inflammation such as the transmural inflammation as seen in fistulas and intestinal fibrosis leading to fibrostenotic strictures, in patients with CD. The interplay between wound healing processes and ECM remodeling also affects the tissue homeostasis in IBD. The interstitial matrix, produced by fibroblasts, holds a very different biology as compared to the epithelial basement membrane in IBD. In combination with integration of wound healing, quantifying the interplay between damage and repair to these sub compartments may provide essential information in IBD patient profiling, mucosal healing and disease management.

Direct and Indirect Roles of Macrophages in Hypertrophic Scar Formation

Hypertrophic scars are pathological scars that result from abnormal responses to trauma, and could cause serious functional and cosmetic disability. To date, no optimal treatment method has been established. A variety of cell types are involved in hypertrophic scar formation after wound healing, but the underlying molecular mechanisms and cellular origins of hypertrophic scars are not fully understood. Macrophages are major effector cells in the immune response after tissue injury that orchestrates the process of wound healing. Depending on the local microenvironment, macrophages undergo marked phenotypic and functional changes at different stages during scar pathogenesis. This review intends to summarize the direct and indirect roles of macrophages during hypertrophic scar formation. The in vivo depletion of macrophages or blocking their signaling reduces scar formation in experimental models, thereby establishing macrophages as positive regulatory cells in the skin scar formation. In the future, a significant amount of attention should be given to molecular and cellular mechanisms that cause the phenotypic switch of wound macrophages, which may provide novel therapeutic targets for hypertrophic scars.

Tumorigenic Interplay Between Macrophages and Collagenous Matrix in the Tumor Microenvironment

The tumor microenvironment is a heterogeneous tissue that in addition to tumor cells, contain tumor-associated cell types such as immune cells, fibroblasts, and endothelial cells. Considerably important in the tumor microenvironment is its noncellular component, namely, the extracellular matrix (ECM). In particular, the collagenous matrix is subjected to significant alterations in its composition and structure that create a permissive environment for tumor growth, invasion, and dissemination. Among tumor-infiltrating immune cells, tumor-associated macrophages (TAMs) are numerous in the tumor stroma and are locally educated to mediate important biological functions that profoundly affect tumor initiation, growth, and dissemination. While the influence of TAMs and mechanical properties of the collagenous matrix on tumor invasion and progression have been comprehensively investigated individually, their interaction within the complex tumor microenvironment was overlooked. This review summarizes accumulating evidence that indicate the existence of an intricate tumorigenic crosstalk between TAMs and collagenous matrix. A better mechanistic comprehension of this reciprocal interplay may open a novel arena for cancer therapeutics.

A complex interplay between the extracellular matrix and the innate immune response to microbial pathogens

The role of the host extracellular matrix (ECM) in infection tends to be neglected. However, the complex interactions between invading pathogens, host tissues and immune cells occur in the context of the ECM. On the pathogen side, a variety of surface and secreted molecules, including microbial surface components recognizing adhesive matrix molecules and tissue-degrading enzymes, are employed that interact with different ECM proteins to effectively establish an infection at specific sites. Microbial pathogens can also hijack or misuse host proteolytic systems to modify the ECM, evade immune responses or process biologically active molecules such as cell surface receptors and cytokines that direct cell behaviour and immune defence. On the host side, the ECM composition and three-dimensional ultrastructure undergo significant modifications, which have a profound impact on the specific signals that the ECM conveys to immune cells at the forefront of infection. Unexpectedly, activated immune cells participate in the remodelling of the local ECM by synthesizing ECM glycoproteins, proteoglycans and collagen molecules. The close interplay between the ECM and the innate immune response to microbial pathogens ultimately affects the outcome of infection. This review explores and discusses recent data that implicate an active role for the ECM in the immune response to infection, encompassing antimicrobial activities, microbial recognition, macrophage activation, phagocytosis, leucocyte population balance, and transcriptional and post-transcriptional regulation of inflammatory networks, and may foster novel antimicrobial approaches.

Age-Related Differential Structural and Transcriptomic Responses in the Hypertensive Heart

While aging is a critical risk factor for heart failure, it remains uncertain whether the aging heart responds differentially to a hypertensive stimuli. Here we investigated phenotypic and transcriptomic differences between the young and aging heart using a mineralocorticoid-excess model of hypertension. Ten-week (“young”) and 36-week (“aging”) mice underwent a unilateral uninephrectomy with deoxycorticosterone acetate (DOCA) pellet implantation (n = 6–8/group) and were followed for 6 weeks. Cardiac structure and function, blood pressure (BP) and the cardiac transcriptome were subsequently examined. Young and aging DOCA mice had high BP, increased cardiac mass, cardiac hypertrophy, and fibrosis. Left ventricular end-diastolic pressure increased in aging DOCA-treated mice in contrast to young DOCA mice. Interstitial and perivascular fibrosis occurred in response to DOCA, but perivascular fibrosis was greater in aging mice. Transcriptomic analysis showed that young mice had features of higher oxidative stress, likely due to activation of the respiratory electron transport chain. In contrast, aging mice showed up-regulation of collagen formation in association with activation of innate immunity together with markers of inflammation including cytokine and platelet signaling. In comparison to younger mice, aging mice demonstrated different phenotypic and molecular responses to hypertensive stress. These findings have potential implications for the pathogenesis of age-related forms of cardiovascular disease, particularly heart failure.

Mapping macrophage polarization over the myocardial infarction time continuum

In response to myocardial infarction (MI), cardiac macrophages regulate inflammation and scar formation. We hypothesized that macrophages undergo polarization state changes over the MI time course and assessed macrophage polarization transcriptomic signatures over the first week of MI. C57BL/6 J male mice (3–6 months old) were subjected to permanent coronary artery ligation to induce MI, and macrophages were isolated from the infarct region at days 1, 3, and 7 post-MI. Day 0, no MI resident cardiac macrophages served as the negative MI control. Whole transcriptome analysis was performed using RNA-sequencing on n = 4 pooled sets for each time. Day 1 macrophages displayed a unique pro-inflammatory, extracellular matrix (ECM)-degrading signature. By flow cytometry, day 0 macrophages were largely F4/80^highLy6C^low resident macrophages, whereas day 1 macrophages were largely F4/80^lowLy6C^high infiltrating monocytes. Day 3 macrophages exhibited increased proliferation and phagocytosis, and expression of genes related to mitochondrial function and oxidative phosphorylation, indicative of metabolic reprogramming. Day 7 macrophages displayed a pro-reparative signature enriched for genes involved in ECM remodeling and scar formation. By triple in situ hybridization, day 7 infarct macrophages in vivo expressed collagen I and periostin mRNA. Our results indicate macrophages show distinct gene expression profiles over the first week of MI, with metabolic reprogramming important for polarization. In addition to serving as indirect mediators of ECM remodeling, macrophages are a direct source of ECM components. Our study is the first to report the detailed changes in the macrophage transcriptome over the first week of MI.

Rapid tissue regeneration induced by intracellular ATP delivery-A preliminary mechanistic study

We have reported a new phenomenon in acute wound healing following the use of intracellular ATP delivery—extremely rapid tissue regeneration, which starts less than 24 h after surgery, and is accompanied by massive macrophage trafficking, in situ proliferation, and direct collagen production. This unusual process bypasses the formation of the traditional provisional extracellular matrix and significantly shortens the wound healing process. Although macrophages/monocytes are known to play a critical role in the initiation and progression of wound healing, their in situ proliferation and direct collagen production in wound healing have never been reported previously. We have explored these two very specific pathways during wound healing, while excluding confounding factors in the in vivo environment by analyzing wound samples and performing in vitro studies. The use of immunohistochemical studies enabled the detection of in situ macrophage proliferation in ATP-vesicle treated wounds. Primary human macrophages and Raw 264.7 cells were used for an in vitro study involving treatment with ATP vesicles, free Mg-ATP alone, lipid vesicles alone, Regranex, or culture medium. Collagen type 1α 1, MCP-1, IL-6, and IL-10 levels were determined by ELISA of the culture supernatant. The intracellular collagen type 1α1 localization was determined with immunocytochemistry. ATP-vesicle treated wounds showed high immunoreactivity towards BrdU and PCNA antigens, indicating in situ proliferation. Most of the cultured macrophages treated with ATP-vesicles maintained their classic phenotype and expressed high levels of collagen type 1α1 for a longer duration than was observed with cells treated with Regranex. These studies provide the first clear evidence of in situ macrophage proliferation and direct collagen production during wound healing. These findings provide part of the explanation for the extremely rapid tissue regeneration, and this treatment may hold promise for acute and chronic wound care.

Molecular imaging of the extracellular matrix in the context of atherosclerosis

This review summarizes the current status of molecular imaging of the extracellular matrix (ECM) in the context of atherosclerosis. Apart from cellular components, the ECM of the atherosclerotic plaque plays a relevant role during the initiation of atherosclerosis and its' subsequent progression. Important structural and signaling components of the ECM include elastin, collagen and fibrin. However, the ECM not only plays a structural role in the arterial wall but also interacts with different cell types and has important biological signaling functions. Molecular imaging of the ECM has emerged as a new diagnostic tool to characterize biological aspects of atherosclerotic plaques, which cannot be characterized by current clinically established imaging techniques, such as X-ray angiography. Different types of molecular probes can be detected in vivo by imaging modalities such as magnetic resonance imaging (MRI), positron emission tomography (PET) and single photon emission computed tomography (SPECT). The modality specific signaling component of the molecular probe provides information about its spatial location and local concentration. The successful introduction of molecular imaging into clinical practice and guidelines could open new pathways for an earlier detection of disease processes and a better understanding of the disease state on a biological level. Quantitative in vivo molecular parameters could also contribute to the development and evaluation of novel cardiovascular therapeutic interventions and the assessment of response to treatment.

Minor collagens of the skin with not so minor functions

The structure and function of the skin relies on the complex expression pattern and organisation of extracellular matrix macromolecules, of which collagens are a principal component. The fibrillar collagens, types I and III, constitute over 90% of the collagen content within the skin and are the major determinants of the strength and stiffness of the tissue. However, the minor collagens also play a crucial regulatory role in a variety of processes, including cell anchorage, matrix assembly, and growth factor signalling. In this article, we review the expression patterns, key functions and involvement in disease pathogenesis of the minor collagens found in the skin. While it is clear that the minor collagens are important mediators of normal tissue function, homeostasis and repair, further insight into the molecular level structure and activity of these proteins is required for translation into clinical therapies.

CCL2-driven inflammation increases mammary gland stromal density and cancer susceptibility in a transgenic mouse model

Background

Macrophages play diverse roles in mammary gland development and breast cancer. CC-chemokine ligand 2 (CCL2) is an inflammatory cytokine that recruits macrophages to sites of injury. Although CCL2 has been detected in human and mouse mammary epithelium, its role in regulating mammary gland development and cancer risk has not been explored.

Methods

Transgenic mice were generated wherein CCL2 is driven by the mammary epithelial cell-specific mouse mammary tumour virus 206 (MMTV) promoter. Estrous cycles were tracked in adult transgenic and non-transgenic FVB mice, and mammary glands collected at the four different stages of the cycle. Dissected mammary glands were assessed for cyclical morphological changes, proliferation and apoptosis of epithelium, macrophage abundance and collagen deposition, and mRNA encoding matrix remodelling enzymes. Another cohort of control and transgenic mice received carcinogen 7,12-Dimethylbenz(a)anthracene (DMBA) and tumour development was monitored weekly. CCL2 protein was also quantified in paired samples of human breast tissue with high and low mammographic density.

Results

Overexpression of CCL2 in the mammary epithelium resulted in an increased number of macrophages, increased density of stroma and collagen and elevated mRNA encoding matrix remodelling enzymes lysyl oxidase (LOX) and tissue inhibitor of matrix metalloproteinases (TIMP)3 compared to non-transgenic controls. Transgenic mice also exhibited increased susceptibility to development of DMBA-induced mammary tumours. In a paired sample cohort of human breast tissue, abundance of epithelial-cell-associated CCL2 was higher in breast tissue of high mammographic density compared to tissue of low mammographic density.

Conclusions

Constitutive expression of CCL2 by the mouse mammary epithelium induces a state of low level chronic inflammation that increases stromal density and elevates cancer risk. We propose that CCL2-driven inflammation contributes to the increased risk of breast cancer observed in women with high mammographic density.

Distinct microenvironmental cues stimulate divergent TLR4-mediated signaling pathways in macrophages

Macrophages exhibit a phenotypic plasticity that enables them to orchestrate specific immune responses to distinct threats. The microbial product lipopolysaccharide (LPS) and the extracellular matrix glycoprotein tenascin-C are released during bacterial infection and tissue injury, respectively, and both activate Toll-like receptor 4 (TLR4). We found that these two TLR4 ligands stimulated distinct signaling pathways in macrophages, resulting in cells with divergent phenotypes. Although macrophages activated by LPS or tenascin-C displayed some common features, including activation of nuclear factor κB and mitogen-activated protein kinase signaling and cytokine synthesis, each ligand stimulated the production of different subsets of cytokines and generated different phosphoproteomic signatures. Moreover, tenascin-C promoted the generation of macrophages that exhibited increased synthesis and phosphorylation of extracellular matrix components, whereas LPS stimulated the production of macrophages that exhibited an enhanced capacity to degrade the matrix. These data reveal how the activation of one pattern recognition receptor by different microenvironmental cues generates macrophage with distinct phenotypes.

Nur77-deficiency in bone marrow-derived macrophages modulates inflammatory responses, extracellular matrix homeostasis, phagocytosis and tolerance

Background

The nuclear orphan receptor Nur77 (NR4A1, TR3, or NGFI-B) has been shown to modulate the inflammatory response of macrophages. To further elucidate the role of Nur77 in macrophage physiology, we compared the transcriptome of bone marrow-derived macrophages (BMM) from wild-type (WT) and Nur77-knockout (KO) mice.

Results

In line with previous observations, SDF-1α (CXCL12) was among the most upregulated genes in Nur77-deficient BMM and we demonstrated that Nur77 binds directly to the SDF-1α promoter, resulting in inhibition of SDF-1α expression. The cytokine receptor CX3CR1 was strongly downregulated in Nur77-KO BMM, implying involvement of Nur77 in macrophage tolerance. Ingenuity pathway analyses (IPA) to identify canonical pathways regulation and gene set enrichment analyses (GSEA) revealed a potential role for Nur77 in extracellular matrix homeostasis. Nur77-deficiency increased the collagen content of macrophage extracellular matrix through enhanced expression of several collagen subtypes and diminished matrix metalloproteinase (MMP)-9 activity. IPA upstream regulator analyses discerned the small GTPase Rac1 as a novel regulator of Nur77-mediated gene expression. We identified an inhibitory feedback loop with increased Rac1 activity in Nur77-KO BMM, which may explain the augmented phagocytic activity of these cells. Finally, we predict multiple chronic inflammatory diseases to be influenced by macrophage Nur77 expression. GSEA and IPA associated Nur77 to osteoarthritis, chronic obstructive pulmonary disease, rheumatoid arthritis, psoriasis, and allergic airway inflammatory diseases.

Conclusions

Altogether these data identify Nur77 as a modulator of macrophage function and an interesting target to treat chronic inflammatory disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2469-9) contains supplementary material, which is available to authorized users.

Immunomodulatory polymeric scaffold enhances extracellular matrix production in cell co-cultures under dynamic mechanical stimulation

Despite the importance of immune cells in regulating the wound healing process following injury, there are few examples of synthetic biomaterials that have the capacity to push the body's immune cells toward pro-regeneration phenotypes, and fewer still that are designed with the intention of achieving this immunomodulatory character. While monocytes and their derived macrophages have been recognized as important contributors to tissue remodeling in vivo, this is primarily believed to be due to their ability to regulate other cell types. The ability of monocytes and macrophages to generate tissue products themselves, however, is currently not well appreciated within the field of tissue regeneration. Furthermore, while monocytes/macrophages are found in remodeling tissue that is subjected to mechanical loading, the effect this biomechanical strain on monocytes/macrophages and their ability to regulate tissue-specific cellular activity has not been understood due to the complexity of the many factors involved in the in vivo setting, hence necessitating the use of controlled in vitro culture platforms to investigate this phenomenon. In this study, human monocytes were co-cultured with human coronary artery smooth muscle cells (VSMCs) on a tubular (3mm ID) degradable polyurethane scaffold, with a unique combination of non-ionic polar, hydrophobic and ionic chemistry (D-PHI). The goal was to determine if such a synthetic matrix could be used in a co-culture system along with dynamic biomechanical stimulus (10% circumferential strain, 1Hz) conditions in order to direct monocytes to enhance tissue generation, and to better comprehend the different ways in which monocytes/macrophages may contribute to new tissue production. Mechanical strain and monocyte co-culture had a complementary and non-mitigating effect on VSMC growth. Co-culture samples demonstrated increased deposition of sulphated glycosaminoglycans (GAGs) and elastin, as well as increases in the release of FGF-2, a growth factor that can stimulate VSMC growth, while dynamic culture supported increases in collagen I and III as well as increased mechanical properties (elastic modulus, tensile strength) vs. static controls. Macrophage polarization toward an M1 state was not promoted by the biomaterial or culture conditions tested. Monocytes/macrophages cultured on D-PHI were also shown to produce vascular extracellular matrix components, including collagen I, collagen III, elastin, and GAGs. This study highlights the use of synthetic biomaterials having immunomodulatory character in order to promote cell and tissue growth when used in tissue engineering strategies, and identifies ECM deposition by monocytes/macrophages as an unexpected source of this new tissue.

STATEMENT OF SIGNIFICANCE

The ability of biomaterials to regulate macrophage activation towards a wound healing phenotype has recently been shown to support positive tissue regeneration. However, the ability of immunomodulatory biomaterials to harness monocyte/macrophage activity to support tissue engineering strategies in vitro holds enormous potential that has yet to be investigated. This study used a monocyte co-culture on a degradable polyurethane (D-PHI) to regulate the response of VSMCs in combination with biomechanical strain in a vascular tissue engineering context. Results demonstrate that immunomodulatory biomaterials, such as D-PHI, that support a desirable macrophage activation state can be combined with biomechanical strain to augment vascular tissue production in vitro, in part due to the novel and unexpected contribution of monocytes/macrophages themselves producing vascular ECM proteins.

Foam Cell Formation In Vivo Converts Macrophages to a Pro-Fibrotic Phenotype

Formation of foam cell macrophages, which sequester extracellular modified lipids, is a key event in atherosclerosis. How lipid loading affects macrophage phenotype is controversial, with evidence suggesting either pro- or anti-inflammatory consequences. To investigate this further, we compared the transcriptomes of foamy and non-foamy macrophages that accumulate in the subcutaneous granulomas of fed-fat ApoE null mice and normal chow fed wild-type mice in vivo. Consistent with previous studies, LXR/RXR pathway genes were significantly over-represented among the genes up-regulated in foam cell macrophages. Unexpectedly, the hepatic fibrosis pathway, associated with platelet derived growth factor and transforming growth factor-β action, was also over-represented. Several collagen polypeptides and proteoglycan core proteins as well as connective tissue growth factor and fibrosis-related FOS and JUN transcription factors were up-regulated in foam cell macrophages. Increased expression of several of these genes was confirmed at the protein level in foam cell macrophages from subcutaneous granulomas and in atherosclerotic plaques. Moreover, phosphorylation and nuclear translocation of SMAD2, which is downstream of several transforming growth factor-β family members, was also detected in foam cell macrophages. We conclude that foam cell formation in vivo leads to a pro-fibrotic macrophage phenotype, which could contribute to plaque stability, especially in early lesions that have few vascular smooth muscle cells.

Macrophage-like U937 cells recognize collagen fibrils with strain-induced discrete plasticity damage

At its essence, biomechanical injury to soft tissues or tissue products means damage to collagen fibrils. To restore function, damaged collagen must be identified, then repaired or replaced. It is unclear at present what the kernel features of fibrillar damage are, how phagocytic or synthetic cells identify that damage, and how they respond. We recently identified a nanostructural motif characteristic of overloaded collagen fibrils that we have termed discrete plasticity. In this study, we have demonstrated that U937 macrophage-like cells respond specifically to overload-damaged collagen fibrils. Tendons from steer tails were bisected, one half undergoing 15 cycles of subrupture mechanical overload and the other serving as an unloaded control. Both halves were decellularized, producing sterile collagen scaffolds that contained either undamaged collagen fibrils, or fibrils with discrete plasticity damage. Matched-pairs were cultured with U937 cells differentiated to a macrophage-like form directly on the substrate. Morphological responses of the U937 cells to the two substrates-and evidence of collagenolysis by the cells-were assessed using scanning electron microscopy. Enzyme release into medium was quantified for prototypic matrix metalloproteinase-1 (MMP-1) collagenase, and MMP-9 gelatinase. When adherent to damaged collagen fibrils, the cells clustered less, showed ruffled membranes, and frequently spread: increasing their contact area with the damaged substrate. There was clear structural evidence of pericellular enzymolysis of damaged collagen-but not of control collagen. Cells on damaged collagen also released significantly less MMP-9. These results show that U937 macrophage-like cells recognize strain-induced discrete plasticity damage in collagen fibrils: an ability that may be important to their removal or repair.

Type VIII collagen mediates vessel wall remodeling after arterial injury and fibrous cap formation in atherosclerosis

Collagens in the atherosclerotic plaque signal regulation of cell behavior and provide tensile strength to the fibrous cap. Type VIII collagen, a short-chain collagen, is up-regulated in atherosclerosis; however, little is known about its functions in vivo. We studied the response to arterial injury and the development of atherosclerosis in type VIII collagen knockout mice (Col8(-/-) mice). After wire injury of the femoral artery, Col8(-/-) mice had decreased vessel wall thickening and outward remodeling when compared with Col8(+/+) mice. We discovered that apolipoprotein E (ApoE) is an endogenous repressor of the Col8a1 chain, and, therefore, in ApoE knockout mice, type VIII collagen was up-regulated. Deficiency of type VIII collagen in ApoE(-/-) mice (Col8(-/-);ApoE(-/-)) resulted in development of plaques with thin fibrous caps because of decreased smooth muscle cell migration and proliferation and reduced accumulation of fibrillar type I collagen. In contrast, macrophage accumulation was not affected, and the plaques had large lipid-rich necrotic cores. We conclude that in atherosclerosis, type VIII collagen is up-regulated in the absence of ApoE and functions to increase smooth muscle cell proliferation and migration. This is an important mechanism for formation of a thick fibrous cap to protect the atherosclerotic plaque from rupture.

Type VIII collagen signals via β1 integrin and RhoA to regulate MMP-2 expression and smooth muscle cell migration

The extracellular matrix signals and regulates the behavior of vascular cells during the pathogenesis of atherosclerosis. Type VIII collagen, a short chain collagen, is scarcely present in normal arteries, but is dramatically upregulated in atherosclerosis and after other types of vascular injury. Cell culture studies have revealed that this protein supports smooth muscle cell (SMC) adhesion and stimulates migration, however little is known about the signaling or the mechanisms by which this occurs. SMCs isolated from wild-type C57BL/6 and type VIII collagen deficient mice were studied using assays to measure chemotactic and haptotactic migration, and remodeling and contraction of 3-dimensional type I collagen gels. Col8(-/-) SMCs exhibited impairments in migration, and a strongly adhesive phenotype with prominent stress fibers, stable microtubules and pronounced central basal focal adhesions. The addition of exogenous type VIII collagen to the Col8(-/-) SMCs rescued the impairments in migration, and restored cytoskeletal architecture so that it was similar to Col8(+/+) cells. We measured elevated levels of active GTP-RhoA in the Col8(-/-) cells, and this too was reversed by treatment with exogenous type VIII collagen. We showed that type VIII collagen normally suppresses RhoA activation through a beta-1 integrin dependent mechanism. MMP-2 levels were reduced in the Col8(-/-) SMCs, and knockdown of MMP-2 in Col8(+/+) SMCs partially recapitulated the decreases in migration and 3D gel contraction seen in Col8(-/-) cells, showing that type VIII collagen-stimulated migration was dependent on MMP-2. Inhibition of Rho restored MMP-2 activity in the Col8(-/-) cells, and partially rescued migration, demonstrating that the elevations in RhoA activity were responsible for the suppression of migration of these cells. In conclusion, we have shown that type VIII collagen signals through beta-1 integrin receptors to suppress RhoA, allowing optimal configuration of the cytoskeleton, and the stimulation of MMP-2-dependent cell migration.

Embryology and anatomy of the vesicoureteric junction with special reference to the etiology of vesicoureteral reflux

Concerning the ureterovesical junction - the region most important for the anti-reflux mechanism - there is still a lot of misunderstanding and misinterpretation with regard to normal fetal development. Data are scarce on possible causes of primary vesicoureteral reflux and on involved mechanisms of the so-called maturation process of refluxing ureteral endings. The ratio of the intravesical ureteral length to the ureteral diameter is obviously lower than assumed so far, as clearly revealed by some studies. Therefore it can be doubted that the length and course of the intravesical ureter is of sole importance in the prevention of reflux. Additionally refluxing intravesical ureteral endings present with dysplasia, atrophy, and architectural derangement of smooth muscle fibers. Besides, a pathologically increased matrix remodeling combined with deprivation of the intramural nerve supply has been confirmed. Consequently, symmetrical narrowing of the very distal ureteral smooth muscle coat creating the active valve mechanism to defend reflux is not achievable. It is apparent that primary congenital vesicoureteral reflux seems to be the result of an abnormality within the ureterovesical junction with an insufficient muscular wrap. Nature is believed to establish much more sophisticated mechanisms than the so-called passive anti-reflux mechanism. Remodeling processes within the ureterovesical junction of refluxing ureteral endings support that maturation itself is nothing else than wound or defect healing and not a restitution of a morphological normal ureterovesical junction. Lacking the nerve supply a restoration of any muscular structure can not be achieved.

Macrophages in skin injury and repair

After recruitment to the wound bed, monocytes differentiate into macrophages. Macrophages play a central role in all stages of wound healing and orchestrate the wound healing process. Their functional phenotype is dependent on the wound microenvironment, which changes during healing, hereby altering macrophage phenotype. During the early and short inflammatory phase macrophages exert pro-inflammatory functions like antigen-presenting, phagocytosis and the production of inflammatory cytokines and growth factors that facilitate the wound healing process. As such, the phenotype of wound macrophages in this phase is probably the classically activated or the so-called M1 phenotype. During the proliferative phase, macrophages stimulate proliferation of connective, endothelial and epithelial tissue directly and indirectly. Especially fibroblasts, keratinocytes and endothelial cells are stimulated by macrophages during this phase to induce and complete ECM formation, reepithelialization and neovascularization. Subsequently, macrophages can change the composition of the ECM both during angiogenesis and in the remodelling phase by release of degrading enzymes and by synthesizing ECM molecules. This suggests an important role for alternatively activated macrophages in this phase of wound healing. Pathological functioning of macrophages in the wound healing process can result in derailed wound healing, like the formation of ulcers, chronic wounds, hypertrophic scars and keloids. However, the exact role of macrophages in these processes is still incompletely understood. For treating wound repair disorders more should be elucidated on the role of macrophages in these conditions, especially their functional phenotype, to find more therapeutic opportunities. This review summarizes macrophage function in skin injury repair, thereby providing more insight in macrophage function in wound healing and possible interventions in this process.

Macrophage diversity in renal injury and repair

Monocyte-derived macrophages can determine the outcome of the immune response and whether this response contributes to tissue repair or mediates tissue destruction. In addition to their important role in immune-mediated renal disease and host defense, macrophages play a fundamental role in tissue remodeling during embryonic development, acquired kidney disease, and renal allograft responses. This review summarizes macrophage phenotype and function in the orchestration of kidney repair and replacement of specialized renal cells following injury. Recent advances in our understanding of macrophage heterogeneity in response to their microenvironment raise new and exciting therapeutic possibilities to attenuate or conceivably reverse progressive renal disease in the context of fibrosis. Furthermore, parallels with pathological processes in many other organs also exist.

Production of type VI collagen by human macrophages: a new dimension in macrophage functional heterogeneity

Macrophages derived from human blood monocytes perform many tasks related to tissue injury and repair. The main effect of macrophages on the extracellular matrix is considered to be destructive in nature, because macrophages secrete metalloproteinases and ingest foreign material as part of the remodeling process that occurs in wound healing and other pathological conditions. However, macrophages also contribute to the extracellular matrix and hence to tissue stabilization both indirectly, by inducing other cells to proliferate and to release matrix components, and directly, by secreting components of the extracellular matrix such as fibronectin and type VIII collagen, as we have recently shown. We now report that monocytes and macrophages express virtually all known collagen and collagen-related mRNAs. Furthermore, macrophages secrete type VI collagen protein abundantly, depending upon their mode of activation, stage of differentiation, and cell density. The primary function of type VI collagen secreted by macrophages appears to be modulation of cell-cell and cell-matrix interactions. We suggest that the production of type VI collagen is a marker for a nondestructive, matrix-conserving macrophage phenotype that could profoundly influence physiological and pathophysiological conditions in vivo.

Monocyte macrophage differentiation in vitro: Fibronectin-dependent upregulation of certain macrophage-specific activities

Transendothelial migration of monocytes followed by their differentiation into macrophages involves interaction of monocytes with subendothelial matrix. The influence of extracellular matrix on monocyte-macrophage differentiation was studied using an in vitro model system with human PBMC maintained on different matrix protein substrata. Upregulation of macrophage specific marker activities such as endocytosis of modified proteins, changes in expression of cell surface antigen, and production of matrix metalloproteinases was studied. Cells maintained on Fibronectin (Fn) showed significantly higher rate of endocytosis and production of MMP2 and MMP9 when compared to other matrix protein substrata. Immunoblot analysis, ELISA, and zymography showed that Fn-dependent upregulation of MMPs was blocked by antibodies to alpha(5)beta(1) integrin indicating that the Fn effect was mediated by integrins. The Fn effect on mo-mPhi was blocked by genistein and herbimycin. As monocytes differentiate to macrophages there was an increase in the rate of production of Fn. These results indicate the influence of the microenvironment of the cell, particularly Fn, on mo-mPhi differentiation and integrin-mediated downstream signaling through focal adhesion kinase and Src type tyrosine kinase is involved in this.

Targeted disruption of Col8a1 and Col8a2 genes in mice leads to anterior segment abnormalities in the eye

Collagen VIII is localized in subendothelial and subepithelial extracellular matrices. It is a major component of Descemet's membrane, a thick basement membrane under the corneal endothelium, where it forms a hexagonal lattice structure; a similar structure, albeit less extensive, may be formed in other basement membranes. We have examined the function of collagen VIII in mice by targeted inactivation of the genes encoding the two polypeptide subunits, Col8a1 and Col8a2. Analysis of these mice reveals no major structural defects in most organs, but demonstrates that type VIII collagen is required for normal anterior eye development, particularly the formation of a corneal stroma with the appropriate number of fibroblastic cell layers and Descemet's membrane of appropriate thickness. Complete lack of type VIII collagen leads to dysgenesis of the anterior segment of the eye: a globoid, keratoglobus-like protrusion of the anterior chamber with a thin corneal stroma. Descemet's membrane is markedly thinned. The corneal endothelial cells are enlarged and reduced in number, and show a decreased ability to proliferate in response to different growth factors in vitro. An important function of collagen VIII may therefore be to generate a peri- or subcellular matrix environment that permits or stimulates cell proliferation.

Migration and growth are attenuated in vascular smooth muscle cells with type VIII collagen-null alleles

OBJECTIVE

Type VIII collagen is upregulated after vascular injury and in atherosclerosis. However, the role of type VIII collagen endogenously expressed by smooth muscle cells (SMCs) and in the context of the vascular matrix microenvironment, which is rich in type I collagen, is not known. To address this, we have compared aortic SMCs from wild-type (WT) mice to SMCs from type VIII collagen-deficient (KO) mice when plated on type I collagen.

METHODS AND RESULTS

Type VIII collagen was upregulated after wounding of WT SMCs. KO SMCs exhibited greater adhesion to type I collagen than WT SMCs (optical density [OD595]=0.458+/-0.044 versus 0.193+/-0.071). By contrast, the WT SMCs spread more (389+/-75% versus 108+/-14% increase in cell area), migrated further (total distance 80.6+/-6.2 microm versus 64.2+/-4.4 microm), and exhibited increased [3H]-thymidine uptake (160,000+/-22,300 versus 63,100+/-12,100 counts per minute) when compared with KO SMCs. Gelatin zymograms showed that WT SMCs expressed latent matrix metalloproteinase 2, whereas KO SMCs did not. Addition of exogenous type VIII collagen returned levels of KO SMC adhesion (OD595=0.316+/-0.038), migration (79.5+/-5.8 microm), and latent matrix metalloproteinase 2 expression to levels comparable to WT SMCs.

CONCLUSIONS

This study suggests that SMCs can modify the matrix microenvironment by producing type VIII collagen, using it to overlay type I collagen, and generating a substrate favorable for migration.

Fetal development of the vesico-ureteric junction, and immunohistochemistry of the ends of refluxing ureters

There is still misunderstanding about the normal fetal development of the vesico-ureteric junction (VUJ), the region that is most important for preventing VUR. There is little information on the causes of primary VUR and on the mechanisms of maturation of refluxing ureteric endings. Some studies show that the ratio of the intravesical ureteric length to diameter is obviously lower than had been assumed. It is doubtful that the length and course of the intravesical ureter is the sole factor in preventing reflux, as previously reported. The intravesical part of refluxing ureters shows dysplasia, atrophy and architectural derangement of smooth muscle fibres. A pathologically increased matrix remodelling combined with deprivation of the intramural nerve supply has been confirmed. Consequently, symmetrical contraction of the distal ureteric smooth muscle coat, creating the active valve mechanism to prevent reflux, is impossible. We reviewed publications using Medline, with the keywords 'human fetal development', 'embryology', 'ureterovesical junction', relevant 'growth data', 'vesico-ureteric reflux', 'children', 'immunohistochemistry', 'extracellular matrix', and 'nerve supply', respectively. Priority was given to articles that correlated specific embryological findings and basic research on possible mechanisms to the genesis and maturation of the VUJ.

Macrophage-specific overexpression of group IIa sPLA2 increases atherosclerosis and enhances collagen deposition

Atherosclerosis is a chronic inflammatory disease of the vessel wall characterized by the accumulation of lipid-laden macrophages and fibrotic material. The initiation of the disease is accompanied by the accumulation of modified lipoproteins in the vessel wall. Group IIa secretory phospholipase A2 (sPLA2 IIa) is a key candidate player in the enzymatic modification of low density lipoproteins. To study the role of sPLA2 IIa in macrophages during atherogenesis, transgenic mice were generated using the human sPLA2 IIa gene and the CD11b promoter. Bone marrow transplantation to LDL receptor-deficient mice was performed to study sPLA2 IIa in atherosclerosis. After 10 weeks of high-fat diet, mice overexpressing sPLA2 IIa in macrophages showed 2.3-fold larger lesions compared with control mice. Pathological examination revealed that sPLA2 IIa-expressing mice had increased collagen in their lesions, independent of lesion size. However, smooth muscle cells or fibroblasts in the lesions were not affected. Other parameters studied, including T-cells and cell turnover, were not significantly affected by overexpression of sPLA2 IIa in macrophages. These data clearly show that macrophage sPLA2 IIa is a proatherogenic factor and suggest that the enzyme regulates collagen production in the plaque and thus fibrotic cap development.

The pathogenesis of atherosclerosis

Worldwide, more people die of the complications of atherosclerosis than of any other cause. It is not surprising, therefore, that enormous resources have been devoted to studying the pathogenesis of this condition. This article attempts to summarize present knowledge on the events that take place within the arterial wall during atherogenesis. Classical risk factors are not dealt with as they are the subjects of other parts of this book. First, we deal with the role of endothelial dysfunction and infection in initiating the atherosclerotic lesion. Then we describe the development of the lesion itself, with particular emphasis on the cell types involved and the interactions between them. The next section of the chapter deals with the events leading to thrombotic occlusion of the atherosclerotic vessel, the cause of heart attack and stroke. Finally, we describe the advantages--and limitations--of current animal models as they contribute to our understanding of atherosclerosis and its complications.

Atherosclerosis and the immune system

The immune system is involved at all stages of the atherosclerotic disease process. Innate immunity, represented by macrophages and other cells, is directly activated by microbial components and possibly also by autologous lipids and proteins. It elicits inflammatory activity, which is a key component of the atherosclerotic lesion. Adaptive immunity is initiated by recognition of disease-related antigens, which include oxidatively modified lipoproteins, heat shock proteins and microbial macromolecules. In the artery wall, adaptive immune recognition mainly leads to Thl effector responses, which are characterized by secretion of proinflammatory cytokines and by activation of macrophages and vascular cells. Therefore, both the innate and adaptive arms of the immune system lead to inflammation in the developing atherosclerotic lesion. Interestingly, several effector pathways of cellular as well as humoral immunity tend to counteract proatherogenic, proinflammatory immunity. The notion that immunity plays an important role in the development of atherosclerosis has focused attention on a number of potential novel targets for intervention based on modulation of such immune responses.

Laser microdissection-based analysis of mRNA expression in human coronary arteries with intimal thickening

Intimal thickening is an early phase of atherosclerosis characterized by differentiation of plaque smooth muscle cells (SMCs) from a contractile to a synthetic phenotype. We used laser microdissection (LMD) plus real-time RT-PCR to quantify mRNAs for calponin-1 and smoothelin, markers of the contractile phenotype, and for serum response factor (SRF), a regulator of SMC differentiation, in intimal and medial SMCs of human coronary arteries with intimal thickening. RNA expression was also analyzed by ISH and protein expression was detected by IHC. LMD plus RT-PCR found similar levels of SRF mRNA in intimal and medial SMCs, while medial mRNA levels for calponin-1 and smoothelin were higher. ISH confirmed that smoothelin mRNA levels in media exceeded those in intima, whereas SRF mRNA levels were similar at both sites. For calponin-1 and smoothelin, protein levels mirrored respective mRNA levels. By contrast, more medial than intimal SRF protein was present. Our results indicate that intimal SMCs exhibit a largely synthetic phenotype, perhaps reflecting lower intimal levels of SRF protein; ISH and LMD plus real-time RT-PCR provide comparable results; as a valuable alternative to ISH, LMD plus RT-PCR allows parallel measurement of several transcripts; and tissue gene expression studies must measure both protein and mRNA levels.

EXTRACELLULAR MATRIX DEGRADATION AND REDUCED NERVE SUPPLY IN REFLUXING URETERAL ENDINGS

PURPOSE

Extracellular matrix (ECM) degrading enzymes and the nerve supply of the ureterovesical junction were investigated using immunohistochemical methods to gain insight into the pattern of refluxing ureteral endings.

MATERIALS AND METHODS

Specimens were obtained from ureterorenal units of 36 children undergoing reflux surgery with a mean age of 62.5 months and 9 age matched controls without reflux. Routine histological paraffin embedded sections were stained for general morphology. Indirect immunohistochemical methods assessing matrix metalloproteinase 1 were used to study the intensity of matrix turnover, and activated macrophage marker CD68 was quantified to describe scavenging of damaged ECM. The intramural neuronal network was explored using nerve specific immunoperoxidase for S-100 protein.

RESULTS

Refluxing ureteral endings demonstrated structural deficiencies of the smooth muscle wrap associated with a 3.8-fold increase of cellular matrix metalloproteinase 1 production and a significant increase of CD68+ macrophages, respectively. The S-100 pattern yielded significant diminution. Lacking B and T lymphocytes in the ECM precluded chronic inflammation.

CONCLUSIONS

Refluxing ureteral endings showed a pathologically increased matrix remodeling combined with deprivation of the intramural nerve supply. Macrophage activation referring to altered morphology was represented by an increased expression of CD68 at the sites of increased ECM turnover.

Expression and Supramolecular Assembly of Recombinant α1(VIII) and α2(VIII) Collagen Homotrimers

Collagen VIII is an extracellular matrix macromolecule comprising two polypeptide chains, alpha1(VIII) and alpha2(VIII), that can form homotrimers in vitro and in vivo. Here, recombinant collagen VIII was expressed to study its supramolecular assembly following secretion. Cells transfected with alpha1(VIII) or alpha2(VIII) assembled and secreted homotrimers that were stable in denaturing conditions and had a molecular mass of approximately 180 kDa on SDS-PAGE gels. Co-transfection with prolyl 4-hydroxylase generated homotrimers with stable pepsin-resistant triple-helical domains. Size fractionation of native recombinant collagen VIII molecules expressed with or without prolyl 4-hydroxylase identified urea-sensitive high molecular mass assemblies eluting in the void volume of a Superose 6HR 10/30 column and urea-resistant assemblies of approximately 700 kDa, all of which were composed of homotrimers. Immunofluorescence analysis highlighted the extracellular deposition of recombinant alpha1(VIII)(3), alpha2(VIII)(3), and co-expressed alpha1(VIII)(3)/alpha2(VIII)(3). Microscopy analysis of recombinant collagen VIII identified rod-like molecules of 134 nm in length that assembled into angular arrays with branching angles of approximately 114 degrees and extensive networks. Based on these data, we propose a model of collagen VIII assembly in which four homotrimers form a tetrahedron stabilized by central interacting C-terminal NC1 trimers. Tetrahedrons may then act as building blocks of three-dimensional hexagonal lattices generated by secondary interactions involving terminal and helical sequences.

Macrophage-specific p53 expression plays a crucial role in atherosclerosis development and plaque remodeling

OBJECTIVE

We first showed that absence of p53 accelerates atherosclerosis development in apoE-deficient mice. In this study, we investigated how macrophage-specific loss of p53 function might modulate atherosclerosis development in LDL receptor-deficient mice, a model for familial hypercholesterolemia.

METHODS AND RESULTS

We transferred bone marrow cells isolated from p53+/+ and p53-/- mice to lethally irradiated LDL receptor-/- mice and evaluated the aortic atherosclerotic lesion areas in the recipients at different times afterward. At 15 weeks and again at 20 weeks, we found larger aortic lesion size in mice receiving p53-/- cells compared with those that received p53+/+ cells. By measuring the rate of bromodeoxyuridine incorporation, we found that the absence of p53 in macrophages stimulates cellular proliferation. In contrast, the rate of apoptosis in the atheromatous lesion was similar in the two groups of mice. Furthermore, we found that the absence of macrophage-specific p53 expression was associated with vulnerable-appearing lesions marked by increased tissue necrosis and reduced collagen deposition.

CONCLUSIONS

p53 plays a crucial role in atherosclerosis lesion development and remodeling, and macrophage-specific p53 deficiency stimulates cellular proliferation leading to a vulnerable-appearing phenotype of lesions in a mouse model of familial hypercholesterolemia.

Rupture of the atherosclerotic plaque: does a good animal model exist?

By its very nature, rupture of the atherosclerotic plaque is difficult to study directly in humans. A good animal model would help us not only to understand how rupture occurs but also to design and test treatments to prevent it from happening. However, several difficulties surround existing models of plaque rupture, including the need for radical interventions to produce the rupture, lack of direct evidence of rupture per se, and absence of convincing evidence of platelet- and fibrin-rich thrombus at the rupture site. At the present time, attention should therefore focus on the processes of plaque breakdown and thrombus formation in humans, whereas the use of animal models should probably be reserved for studying the function of particular genes and for investigating isolated features of plaques, such as the relationship between cap thickness and plaque stability.

Expression of transcription factors and matrix genes in response to serum stimulus in vascular smooth muscle cells

During atherogenesis vascular smooth muscle cells are converted from a contractile into a synthetic phenotype characterized by enhanced matrix production. The transcription factors Gax and GATA-6 are considered negative, and Oct-1 positive regulators of the synthetic phenotype. Since the phenotype transition can be induced by culturing the cells with serum, we followed the expression of Gax, GATA-6 and Oct-1, integrins and matrix genes in quiescent porcine vascular smooth muscle cells after serum application. Comparisons were made between enzymatically released primary smooth muscle cells and cells grown out from explants of the medial layer of porcine aorta. The serum-mediated down-regulation of Gax was more intense than that of GATA-6, and stronger in explant-derived than in primary cells. Serum was without influence on the expression of Oct-1. Changes in the expression of the transcription factors preceded the induction of integrin alpha2 and the down-regulation of decorin, while mRNAs for laminin beta1 and osteopontin rose immediately after serum stimulation. Primary cells reacted more rapidly than explant cells with respect to changes in laminin isoforms. Studies with a Gax-expressing adenovirus indicated that among all the gene products tested only the expression of integrin alpha2 responded to Gax induction. Thus, our data show that i) Gax should be considered a transcription factor being directly responsible for only few aspects of the phenotypic conversion of smooth muscle cells and that ii) explant cells may represent a subpopulation of smooth muscle cells, which differ from the total population of smooth muscle cells, as obtained in primary culture, in their response to serum stimuli.

Vascular collagens: spotlight on the role of type VIII collagen in atherogenesis

Collagens play a central role in maintaining the integrity and stability of the undiseased as well as of the atherosclerotic vessel wall. An imbalanced metabolism may lead to uncontrolled collagen accumulation reducing vessel wall velocity, frequently resulting in arterial occlusion or thrombosis. A reduced production of collagen and its uncontrolled degradation may affect the stability of the vessel wall and especially of the atherosclerotic plaques by making them prone to rupture and aneurysm. This review presents an overview on the four groups of vascular collagens and on their role in atherogenesis. The major focus was to highlight the extraordinary role and importance of the short chain network forming type VIII collagen in the extracellular matrix of undiseased arteries and of atherosclerotic plaques. The molecular structure of type VIII collagen, its cellular origin, its implication in atherogenesis, its temporal and spatial expression patterns in human and experimental models of atherogenesis, the factors modulating its expression, and--not at least--its potential function is discussed.

Expression of the ATP-binding cassette transporter gene ABCG1 (ABC8) in Tangier disease

Several members of the ATP-binding cassette (ABC) transporter family are involved in cholesterol efflux from cells. A defect in one member, ABCA1, results in Tangier disease, a condition characterized by cholesterol accumulation in macrophages and virtual absence of mature circulating high-density lipoproteins. Expression of a second member, ABCG1, is increased by cholesterol-loading in human macrophages. We now show that ABCG1, which we identified by differential display RT-PCR in foamy macrophages, is overexpressed in macrophages from patients with Tangier disease compared to control macrophages. On examination by confocal laser scanning microscopy, ABCG1 was present in perinuclear structures within the cell. In addition, a combination of in situ hybridization and indirect immunofluorescence microscopy revealed that ABCG1 is expressed in foamy macrophages within the atherosclerotic plaque. These data indicate that not only ABCA1 but also ABCG1 may play a role in the cholesterol metabolism of macrophages in vitro and in the atherosclerotic plaque.

The extracellular matrix can regulate vascular cell migration, proliferation, and survival: relationships to vascular disease

The extracellular matrix (ECM) of the normal artery wall is a collection of fibrous proteins and associated glycoproteins embedded in a hydrated ground substance of glycosaminoglycans and proteoglycans. These distinct molecules are organized into a highly ordered network that are closely associated with the vascular cells that produce them. In addition to providing the architectural framework for the artery wall that imparts mechanical support and viscoelasticity, the ECM can regulate the behaviour of vascular cells, including their ability to migrate, proliferate and survive injury. The composition of the ECM is different within intimal lesions of atherosclerosis, which are composed of monocytes and lymphocytes from the circulation and smooth muscle cells (SMC) that migrate from the media to the intima (Ross 1993, 1999), and these differences may contribute to the altered phenotype of vascular cells within lesions. This review will briefly outline the ECM changes observed in atherosclerosis and restenosis and the potential relationship of these changes to altered vascular cell functions.