Åsa Lidén

Edema and fluid dynamics in connective tissue remodelling

The review describes the role of loose connective tissues with focus on transcapillary exchange and edema formation with relevance for inflammation, fibrosis and tumors. Based on studies in these tissues, comparisons are made to the fibrotic processes in the heart.

A secreted collagen and fibronectin-binding streptococcal protein modulates cell-mediated collagen gel contraction and interstitial fluid pressure

Fibroblast-mediated collagen gel contraction depends on collagen-binding β1 integrins. Perturbation of these integrins reveals an alternative contraction process that is integrin αVβ3-dependent and platelet-derived growth factor (PDGF) BB-stimulated. Connective tissue cells actively control interstitial fluid pressure (IFP), and inflammation-induced lowering of IFP provides a driving force for edema formation. PDGF-BB normalizes a lowered IFP by an αVβ3-dependent process. A potential modulation of IFP by extracellular matrix-binding bacterial proteins has previously not been addressed. The fibronectin (FN)-binding protein FNE is specifically secreted by the highly virulent Streptococcus equi subspecies equi. FNE bound FN and native collagen type I with Kd values of ∼20 and ∼50 nm determined by solid-phase binding assays. Rotary shadowing revealed a single FNE binding site located at on average 122 nm from the C terminus of procollagen type I. FNE induced αVβ3-mediated contraction by C2C12 cells in a concentration-dependent manner having a maximal effect at ∼100 nm. This activity of FNE required cellular FN, and FNE acted synergistically to added plasma FN or PDGF-BB. FNE enhanced binding of soluble FN to immobilized collagen, and conversely the binding of collagen to immobilized FN. Marked bell-shaped concentration dependences for these interactions suggest that FNE forms a bridge between FN and collagen. Finally, FNE normalized dermal IFP lowered by anaphylaxis. Our data suggest that secreted FNE normalized lowering of IFP by stimulating connective tissue cell contraction.

Fibroblast EXT1-Levels Influence Tumor Cell Proliferation and Migration in Composite Spheroids

Background Stromal fibroblasts are important determinants of tumor cell behavior. They act to condition the tumor microenvironment, influence tumor growth, support tumor angiogenesis and affect tumor metastasis. Heparan sulfate proteoglycans, present both on tumor and stromal cells, interact with a large number of ligands including growth factors, their receptors, and structural components of the extracellular matrix. Being ubiquitously expressed in the tumor microenvironment heparan sulfate proteoglycans are candidates for playing central roles in tumor-stroma interactions. The objective of this work was to investigate the role of heparan sulfate expressed by stromal fibroblasts in modulating the growth of tumor cells and in controlling the interstitial fluid pressure in a 3-D model. Methodology/Principal Findings We generated spheroids composed of fibroblasts alone, or composite spheroids, composed of fibroblasts and tumor cells. Here we show that stromal fibroblasts with a mutation in the heparan sulfate elongating enzyme Ext1 and thus a low heparan sulfate content, formed composite fibroblast/tumor cell spheroids with a significant lower interstitial fluid pressure than corresponding wild-type fibroblast/tumor cell composite spheroids. Furthermore, immunohistochemistry of composite spheroids revealed that the cells segregated, so that after 6 days in culture, the wild-type fibroblasts formed an inner core and the tumor cells an outer layer of cells. For composite spheroids containing Ext1-mutated fibroblasts this segregation was less obvious, indicating impaired cell migration. Analysis of tumor cells expressing the firefly luciferase gene revealed that the changes in tumor cell migration in mutant fibroblast/tumor cell composite spheroids coincided with a lower proliferation rate. Conclusions/Significance This is the first demonstration that stromal Ext1-levels modulate tumor cell proliferation and affect the interstitial fluid pressure in a 3-D spheroid model. Learning how structural changes in stromal heparan sulfate influence tumor cells is essential for our understanding how non-malignant cells of the tumor microenvironment influence tumor cell progression.

Integrin αvβ3 acts downstream of insulin in normalization of interstitial fluid pressure in sepsis and in cell-mediated collagen gel contraction

The administration of insulin is recommended to patients with severe sepsis and hyperglycemia. Previously, we demonstrated that insulin may have direct anti-inflammatory properties and counteracted fluid losses from the circulation by normalizing the interstitial fluid pressure (P(IF)). P(IF) is one of the Starling forces determining fluid flux over the capillary wall, and a lowered P(IF) is one of the driving forces in early edema formation in inflammatory reactions. Here we demonstrate that insulin restores a lipopolysaccharide (LPS)-lowered P(IF) via a mechanism involving integrin alpha(v)beta(3). In C57 black mice (n = 6), LPS lowered P(IF) from -0.2 +/- 0.2 to -1.6 +/- 0.3 (P < 0.05) and after insulin averaged -0.8 +/- 0.2 mmHg (P = 0.098 compared with after LPS). Corresponding values in wild-type BALB/c mice (n = 5) were -0.8 +/- 0.1, -2.1 +/- 0.3 (P < 0.05), and -0.8 +/- 0.3 mmHg (P < 0.05 compared with LPS) after insulin administration. In BALB/c integrin beta(3)-deficient (beta(3)(-/-)) mice (n = 6), LPS lowered P(IF) from -0.1 +/- 0.2 to -1.5 +/- 0.3 mmHg (P < 0.05). Insulin did not, however, restore P(IF) in these mice (averaged -1.7 +/- 0.3 mmHg after insulin administration). Cell-mediated collagen gel contraction can serve as an in vitro model for in vivo measurements of P(IF). Insulin induced alpha(v)beta(3)-integrin-dependent collagen gel contraction mediated by C2C12 cells. Our findings suggest a beneficiary effect of insulin for patients with sepsis with regard to the fluid balance, and this effect may in part be due to a normalization of P(IF) by a mechanism involving the integrin alpha(v)beta(3).

Increased microvascular permeability in mice lacking Epac1 (RapGef3)

Maintenance of the blood and extracellular volume requires tight control of endothelial macromolecule permeability, which is regulated by cAMP signalling. This study probes the role of the cAMP mediators rap guanine nucleotide exchange factor 3 and 4 (Epac1 and Epac2) for in vivo control of microvascular macromolecule permeability under basal conditions.

The Streptococcal Collagen-binding Protein CNE Specifically Interferes with αvβ3-mediated Cellular Interactions with Triple Helical Collagen

Collagen fibers expose distinct domains allowing for specific interactions with other extracellular matrix proteins and cells. To investigate putative collagen domains that govern integrin αVβ3-mediated cellular interactions with native collagen fibers we took advantage of the streptococcal protein CNE that bound native fibrillar collagens. CNE specifically inhibited αVβ3-dependent cell-mediated collagen gel contraction, PDGF BB-induced and αVβ3-mediated adhesion of cells, and binding of fibronectin to native collagen. Using a Toolkit composed of overlapping, 27-residue triple helical segments of collagen type II, two CNE-binding sites present in peptides II-1 and II-44 were identified. These peptides lack the major binding site for collagen-binding β1 integrins, defined by the peptide GFOGER. Peptide II-44 corresponds to a region of collagen known to bind collagenases, discoidin domain receptor 2, SPARC (osteonectin), and fibronectin. In addition to binding fibronectin, peptide II-44 but not II-1 inhibited αVβ3-mediated collagen gel contraction and, when immobilized on plastic, supported adhesion of cells. Reduction of fibronectin expression by siRNA reduced PDGF BB-induced αVβ3-mediated contraction. Reconstitution of collagen types I and II gels in the presence of CNE reduced collagen fibril diameters and fibril melting temperatures. Our data indicate that contraction proceeded through an indirect mechanism involving binding of cell-produced fibronectin to the collagen fibers. Furthermore, our data show that cell-mediated collagen gel contraction does not directly depend on the process of fibril formation.

Control of Interstitial Fluid Homeostasis: Roles of Growth Factors and Integrins

Fluid and solutes are constantly filtered from the blood stream to the surrounding loose interstitial connective tissue. This flow of fluid has importance for, e.g., cellular metabolism and immunosurveillance. The filtered fluid is moved through the tissues into the lymphatic vessels, which eventually return fluid and solutes back into the blood circulation. The driving force for the filtration results from differences between fluid pressures in the blood vessel and loose connective tissue. The Starling equation, JV = ΔPK, describes fluid filtration (JV) across a capillary wall (for a review, see Ref.1). K is a constant expressing capillary area and permeability. ΔP is the differences in the colloid osmotic pressures in plasma (COPc) and interstitial fluid (COPif), and between capillary hydrostatic pressure (Pc) and interstitial fluid pressure (Pif) according to: ΔPP = (Pc − Pif ) − σ (COPc − COPif), where σ is the plasma protein reflection coefficient. Normally, σ is close to 1, reflecting the low leakage of plasma proteins from normal blood vessels. The higher concentration of diffusible proteins in plasma compared to interstitial fluid together with the properties of the capillary wall causes a higher COPc than COPif, generating a pressure difference that tends to keep fluid within the vessels. Inflammatory processes result in an increased vascular permeability for plasma proteins with a lowering of σ that may cause edema formation since σ (COPc − COPif) is lowered.

Lowered albumin extravasation rate in heart but not in other organs in β3‐integrin‐deficient mice

Aim:  The vascular protein permeability is dependent on the integrity of the vascular wall. The heart capillaries in male mice lacking β3 integrins have an immature phenotype. Previously, we have demonstrated a role for αvβ3 integrins in control of interstitial fluid pressure (Pif) and thereby in the fluid flux during inflammation. We wanted to explore a possible role for αvβ3 integrins in controlling capillary protein permeability during control situation and inflammation.

Integrin αVβ3 can substitute for collagen‐binding β1‐integrins in vivo to maintain a homeostatic interstitial fluid pressure

What is the central question of this study? Collagen‐binding β1‐integrins function physiologically in cellular control of dermal interstitial fluid pressure (PIF) in vivo and thereby participate in control of extravascular fluid volume. During anaphylaxis, simulated by injection of compound 48/80, integrin αVβ3 takes over this physiological function. Here we addressed the question whether integrin αVβ3 can replace collagen‐binding β1‐integrin to maintain a long‐term homeostatic PIF. What is the main finding and its importance? Mice lacking the collagen‐binding integrin α11β1 show a complex dermal phenotype with regard to the interstitial physiology apparent in the control of PIF. Notably dermal PIF is not lowered with compound 48/80 in these animals. Our present data imply that integrin αVβ3 is the likely candidate that has taken over the role of collagen‐binding β1‐integrins for maintaining a steady‐state homeostatic PIF. A better understanding of molecular processes involved in control of PIF is instrumental for establishing novel treatment regimens for control of oedema formation in anaphylaxis and septic shock.