Myofibroblast or smooth muscle: do in vitro systems adequately replicate tissue smooth muscle?

Inhibition of longevity regulator PAPP-A modulates tissue homeostasis via restraint of mesenchymal stromal cells

Pregnancy-associated plasma protein-A (PAPP-A) is a secreted metalloprotease that increases insulin-like growth factor (IGF) availability by cleaving IGF-binding proteins. Reduced IGF signaling extends longevity in multiple species, and consistent with this, PAPP-A deletion extends lifespan and healthspan; however, the mechanism remains unclear. To clarify PAPP-A's role, we developed a PAPP-A neutralizing antibody and treated adult mice with it. Transcriptomic profiling across tissues showed that anti-PAPP-A reduced IGF signaling and extracellular matrix (ECM) gene expression system wide. The greatest reduction in IGF signaling occurred in the bone marrow, where we found reduced bone, marrow adiposity, and myelopoiesis. These diverse effects led us to search for unifying mechanisms. We identified mesenchymal stromal cells (MSCs) as the source of PAPP-A in bone marrow and primary responders to PAPP-A inhibition. Mice treated with anti-PAPP-A had reduced IGF signaling in MSCs and dramatically decreased MSC number. As MSCs are (1) a major source of ECM and the progenitors of ECM-producing fibroblasts, (2) the originating source of adult bone, (3) regulators of marrow adiposity, and (4) an essential component of the hematopoietic niche, our data suggest that PAPP-A modulates bone marrow homeostasis by potentiating the number and activity of MSCs. We found that MSC-like cells are the major source of PAPP-A in other tissues also, suggesting that reduced MSC-like cell activity drives the system-wide reduction in ECM gene expression due to PAPP-A inhibition. Dysregulated ECM production is associated with aging and drives age-related diseases, and thus, this may be a mechanism by which PAPP-A deficiency enhances longevity.

Smooth muscle: a stiff sculptor of epithelial shapes

Smooth muscle is increasingly recognized as a key mechanical sculptor of epithelia during embryonic development. Smooth muscle is a mesenchymal tissue that surrounds the epithelia of organs including the gut, blood vessels, lungs, bladder, ureter, uterus, oviduct and epididymis. Smooth muscle is stiffer than its adjacent epithelium and often serves its morphogenetic function by physically constraining the growth of a proliferating epithelial layer. This constraint leads to mechanical instabilities and epithelial morphogenesis through buckling. Smooth muscle stiffness alone, without smooth muscle cell shortening, seems to be sufficient to drive epithelial morphogenesis. Fully understanding the development of organs that use smooth muscle stiffness as a driver of morphogenesis requires investigating how smooth muscle develops, a key aspect of which is distinguishing smooth muscle-like tissues from one another in vivo and in culture. This necessitates a comprehensive appreciation of the genetic, anatomical and functional markers that are used to distinguish the different subtypes of smooth muscle (for example, vascular versus visceral) from similar cell types (including myofibroblasts and myoepithelial cells). Here, we review how smooth muscle acts as a mechanical driver of morphogenesis and discuss ways of identifying smooth muscle, which is critical for understanding these morphogenetic events.This article is part of the Theo Murphy meeting issue 'Mechanics of Development'.

Bradykinin-induced asthmatic fibroblast/myofibroblast activities via bradykinin B2 receptor and different MAPK pathways

Bradykinin drives normal lung fibroblasts into myofibroblasts, induces fibroblast proliferation and activates mitogen activated protein kinase pathways (MAPK) but its effects on bronchial fibroblasts from asthmatics (HBAFb) have not been yet studied. We studied bradykinin-induced fibroblast proliferation and differentiation and the related intracellular mechanisms in HBAFb compared to normal bronchial fibroblasts (HNBFb). Bradykinin-stimulated HBAFb and HNBFb were used to assess: bradykinin B2 receptor expression by Western blot analysis; cell proliferation by [(3)H] thymidine incorporation; α-smooth muscle actin (SMA) expression/polymerization by Western blot and immunofluorescence; epidermal growth factor (EGF) receptor, extracellular-regulated kinase (ERK) 1/2 and p38 MAPK activation by immunoprecipitation and Western blot, respectively. Constitutive bradykinin B2 receptor and α-SMA expression was higher in HBAFb as compared to HNBFb. Bradykinin increased bradykinin B2 receptor expression in HBAFb. Bradykinin, via bradykinin B2 receptor, significantly increased fibroblast proliferation at lower concentration (10(-11)M) and α-SMA expression/polymerization at higher concentration (10(-6)M) in both cells. Bradykinin increased ERK1/2 and p38 phosphorylation via bradykinin B2 receptor; EGF receptor inhibitor AG1478 and panmetalloproteinase inhibitor GM6001 blocked bradykinin-induced ERK1/2 activation but not p38 phosphorylation. Bradykinin, via bradykinin B2 receptor, induced EGF receptor phosphorylation that was suppressed by AG1478. In HBAFb AG1478, GM6001, the ERK1/2-inhibitor U0126 and the p38 inhibitor SB203580 suppressed bradykinin-induced cell proliferation, but only SB203580 reduced myofibroblast differentiation. These data indicate that bradykinin is actively involved in asthmatic bronchial fibroblast proliferation and differentiation, through MAPK pathways and EGF receptor transactivation, by which bradykinin may contribute to airway remodeling in asthma, opening new horizons for potential therapeutic implications in asthmatic patients.

Mast cells infiltrate the esophageal smooth muscle in patients with eosinophilic esophagitis, express TGF-β1, and increase esophageal smooth muscle contraction

BACKGROUND

Increased numbers of mast cells are present in the esophageal epithelium in patients with eosinophilic esophagitis (EE). However, mast cell infiltration into the esophageal lamina propria (LP) and smooth muscle (SM) and the effects of their products on SM function has not been determined.

OBJECTIVE

We investigated mast cell localization and characterization in esophageal SM, the functional significance of mast cell TGF-β1 expression to contraction of human esophageal smooth muscle (HESM) cells in vitro, and the effect of topical corticosteroids on the number of tryptase-positive (MC(T)) and chymase-positive (MC(C)) mast cells in patients with EE.

METHODS

MC(T)- and MC(C)-positive mast cell numbers were quantitated in the epithelium, the LP before and after topical corticosteroid therapy, and the muscularis mucosa in patients with EE and control subjects by using immunohistology. Double immunofluorescence was used to assess mast cell production of TGF-β1. The ability of TGF-β1 to influence HESM cell contractility was assessed in vitro.

RESULTS

In the SM in patients with EE, significantly increased numbers of MC(T)- and TGF-β1-positive cells (but only low levels of eosinophils) were detected compared with those seen in control subjects. MC(T) expressed TGF-β1, which increased the contractility of cultured primary HESM cells in vitro. Topical corticosteroid therapy in patients with EE significantly reduced epithelial MC(T) numbers but not LP tryptase-chymase-positive mast cell numbers.

CONCLUSIONS

MC(T) numbers, rather than eosinophil numbers, are increased in the SM in patients with EE, express TGF-β1, and increase the contractility of HESM cells in vitro. As such, mast cells localized to SM in patients with EE might modulate esophageal contractility.

Can lineage-specific markers be identified to characterize mesenchyme-derived cell populations in the human airways?

Mesenchyme-derived cells in the airway wall including airway smooth muscle cells, fibroblasts, and myofibroblasts are known to play important roles in airway remodeling. The lack of specific phenotypical markers makes it difficult to define these cell populations in primary cultures. Most relevant studies to date have used animal airway tissues, vascular tissues, or transformed cell lines with only limited studies attempting to phenotypically characterize human airway mesenchymal cells. The objectives of this study were to evaluate reported markers and identify novel markers to define these cell types. We could not identify any specific marker to define these cell populations in vitro that permitted unequivocal identification using immunocytochemistry. However, characteristic filamentous alpha-smooth muscle actin distribution was observed in a significant ( approximately 25%) proportion of human airway smooth muscle cells, whereas this was not observed in airway fibroblasts. A significantly higher proportion of airway fibroblasts expressed alpha(1)- and alpha(2)-integrin receptors compared with human airway smooth muscle cells as assessed by fluorescence activated cell sorting. Global gene expression profiling identified aldo-keto reductase 1C3 (AKR1C3) and cathepsin K as being novel markers to define airway smooth muscle cells, whereas integrin-alpha(8) (ITGA8) and thromboxane synthase 1 (TBXAS1) were identified as novel airway fibroblast-specific markers, and these findings were validated by RT-PCR. Ex vivo studies in human airway tissue sections identified high-molecular weight caldesmon and alpha-smooth muscle actin as being expressed in smooth muscle bundles, whereas ITGA8 and TBXAS1 were absent from these.

Collagen I and thrombin activate MMP-2 by MMP-14-dependent and -independent pathways: implications for airway smooth muscle migration

Increased proinflammatory mediators and ECM deposition are key features of the airways in asthma. Matrix metalloproteinases (MMPs) are produced by airway smooth muscle (ASM) cells and have multiple roles in inflammation and tissue remodeling. We hypothesized that components of the asthmatic airway would stimulate MMP production and activation by ASM and contribute to airway remodeling. We measured human ASM-derived MMP mRNA, protein, and activity by real-time RT-PCR, zymography, Western blotting, and MMP activity assay. Collagen I and thrombin caused a synergistic increase in MMP-2 protein and total MMP activity but paradoxically decreased MMP-2 mRNA. Additionally, collagen I activated MMP-2 in culture supernatants independent of the cell surface. Together, collagen I and thrombin strongly enhanced MMP-14 mRNA and protein but had no effect individually, suggesting increased MMP-14, the activating protease for MMP-2, may be partially responsible for MMP-2 activation. Furthermore, collagen I reduced tissue inhibitor of metalloproteinase-2 protein (TIMP-2). We examined the role of MMPs in functions of ASM related to airway remodeling and found migration and proliferation were MMP dependent, whereas adhesion and apoptosis were not. Ilomastat inhibited migration by 25%, which was also inhibited by TIMPs 1-4 and increased by the MMP-2 activator thrombin. These in vitro findings suggest that the environment within the airways of patients with asthma enhances MMP-2 and -14 protein and activity by a complex interaction of transcriptional and posttranscriptional mechanisms, which may contribute to ASM migration.