Reduced granulation tissue and wound strength in the absence of α11β1 integrin

Abstract

Previous wound healing studies have failed to define a role for either α1β1 or α2β1 integrin in fibroblast-mediated wound contraction, suggesting the involvement of another collagen receptor in this process. Our previous work demonstrated that the integrin subunit α11 is highly induced during wound healing both at the mRNA and protein level, prompting us to investigate and dissect the role of the integrin α11β1 during this process. Therefore, we used mice with a global ablation of either α2 or α11 or both integrin subunits and investigated the repair of excisional wounds. Analyses of wounds demonstrated that α11β1 deficiency results in reduced granulation tissue formation and impaired wound contraction, independently of the presence of α2β1. Our combined in vivo and in vitro data further demonstrate that dermal fibroblasts lacking α11β1 are unable to efficiently convert to myofibroblasts, resulting in scar tissue with compromised tensile strength. Moreover, we suggest that the reduced stability of the scar is a consequence of poor collagen remodeling in α11(-/-) wounds associated with defective transforming growth factor-β-dependent JNK signaling.

Figure 1

Ablation of α11β1 impairs wound healing. (a) Hematoxylin and eosin staining of mid-wound sections from α11^+/+, α2^−/−, α11^−/− and α2^−/−//α11^−/− mice at day 7 after wounding. (b) Histomorphometry of granulation tissues. (c) Distance between panniculus carnosus edges and (d) left and right wound edges. Each symbol represents one wound and 3–5 sections/wound were analyzed. Data were evaluated by one-way analysis of variance and Tukey's multiple comparison test. epi, new epidermis; g, granulation tissue. Scale bar=500 μm.

Figure 2

Cell migration is impaired in α11^+/+ fibroblasts. (a) ³⁵S metabolic labeling and immunoprecipitation of integrins in murine dermal fibroblasts. (b) Contribution of α11 integrin to dermal fibroblast adhesion. Integrin blocking antibodies (anti-β1 or anti-α2) were added (10 μg ml⁻¹). ∅ represents untreated cells. Adhesion to fibronectin was used as control. (c) Random migration of dermal fibroblasts was analyzed on type I collagen or fibronectin. Cell motility was quantified by measuring the cell trajectory of single cells; each symbol represents one cells. Average trajectories (∅) were calculated for each condition. (*P<0.05; ***P<0.001; NS, not significantly different; mean ±SD). O.D. optical density.

Figure 3

Ablation of α11β1 results in significant reduction in myofibroblast differentiation. (a) Immunohistochemical staining of mid-wound sections at 7 days after injury with antibodies directed to α-SMA (red). Scale bar = 500 μm. (b) α-SMA-positive area was quantified by histomorphometry. Each symbol represents one wound, and 3–5 sections per wound were evaluated. (c) Western immunoblotting of wound extracts harvested 7 days after injury (4 mm punch biopsies from the wound center) indicating levels of α-SMA. (d) α-SMA expression by primary fibroblasts embedded in attached collagen lattices was analyzed by western immunoblotting. (e) Quantification of α-SMA signals in (d). (f) Primary fibroblasts on collagen I (100 μg ml⁻¹) were treated with SB505124 (10 μM) or SIS3 (10 μM) for 48 hours. Protein expression was analyzed by western immunoblotting and quantified by densitometry. α-SMA, α-smooth muscle actin.

Figure 4

Ablation of α11β1 impairs collagen remodeling. (a) Tensile strength of incisional wounds determined at 16d after injury. Fmax depicts ultimate force applied to skin strips before the scar ruptured. Each symbol represents one incisional wound. (b) Skin strips were excised using the punch with the indicated shape. Arrow indicates position of the scar. (c) Sirius red staining of GTs analyzed by polarized light microscopy. Colors were inverted and split into single channels to separatly assess the amount of green (thin) fibrils. (d) Quantification of the green fibrils from (c). (e) Fibroblasts were allowed to remodel collagen for 72 h. (f) Western immunoblotting of α11 expression after α11 knock-in (KI) in α11^−/− fibroblasts. (g) Effect of α11 knock-in on collagen remodeling (***P<0.001; mean ±SD). GT, granulation tissue; GAPDH, glyceraldehyde-3-phosphate dehydrogenase.

Figure 5

Collagen remodeling is dependent on TGF-β-mediated JNK signaling. (a) Fibroblasts were incorporated into collagen lattices and allowed to contract for 24 hours in serum-free DMEM or in the presence of TGF-β1 (5 ng ml⁻¹). (b) Effect of signaling inhibitors (SB505124, 10 μM or SP600125, 25 μM) on collagen remodeling stimulated with 2% serum. (c) JNK activation was evaluated during collagen remodeling. (d) Effect of SB505124 on JNK phosphorylation in α11^+/+ fibroblasts during collagen remodeling. (e) c-jun activition was assessed after fibroblast co-transfection with DN JNK constructs. UV light was used to activate the JNK pathway. (f) Effect of DN JNK on collagen remodeling stimulated with 2% serum. (g) JNK^−/− MEFs were allowed to remodel floating collagen gels for 24 hours in the presence of 2% serum (**P<0.01; ***P<0.001; mean ±SD). DN, dominant negative; TGF-β, transforming growth factor-β.