Plasminogen mediates the pathological effects of urokinase-type plasminogen activator overexpression

Abstract

Increased expression of urokinase-type plasminogen activator (uPA) and its receptor (uPAR) is associated with different pathological conditions. Both uPAR-mediated signaling and plasmin-catalyzed extracellular proteolysis may contribute to pathogenesis. To evaluate the involvement of plasminogen in such circumstances, we have taken advantage of transgenic mouse models in which overexpression of uPA and/or uPAR in enamel epithelium, basal epidermis, and hair follicles leads to a pathological phenotype; uPA transgenic mice have chalky-white incisors and, when uPAR is co-expressed, develop extensive alopecia, epidermal thickening, and subepidermal blisters. We report here that when these transgenic mice were backcrossed into a plasminogen-deficient (Plg-/-) background, the dental and skin phenotypes appeared completely normal. Heterozygous Plg+/- transgenic mice exhibited a haplo-insufficiency, with an intermediate or normal phenotype. These results do not argue in favor of a role for uPAR-mediated signaling in our experimental model; rather, they demonstrate an essential, dose-dependent, requirement for plasminogen in uPA-mediated tissue alterations. They also support the hypothesis that plasminogen could play a part in certain skin diseases.

Figure 1

A: Gross appearance of incisors of 3 month-old littermates. B and C: Scanning electron micrographs of the labial surface of incisors. In K5-uPA, Plg+/+ incisors, a few plaques of granular material (arrow) cover the labial surface whereas both K5-uPA, Plg+/− and K5-uPA, Plg−/− incisor surface is uniformly covered by smooth material as is the control Plg−/− incisor surface. Bar, 160 μm in (B) and 20 μm in (C).

Figure 2

Labial side of cross-section of non-decalcified incisors. Lack of enamel covering the dentin (asterisks) in K5-uPA, Plg+/+ incisors. Presence of an enamel layer (between two white arrowheads) of equal thickness in K5-uPA, Plg+/−, K5-uPA, Plg−/− and control (wild-type) littermate incisors. Bar, 60 μm.

Figure 3

Hematoxylin-eosin staining of paraffin sections of enamel organ of 1-day-old mice. In K5-uPA, Plg+/+ mice, ameloblast layer (asterisks) is disorganized and large crystals (arrow) are deposited on the dentin surface. In K5-uPA, Plg +/−, K5-uPA, Plg−/− and control (Plg−/− and wild-type) mice, ameloblasts are well aligned and a regular layer of enamel (between two arrowheads) covers the dentin surface. Bar, 60 μm.

Figure 4

Phenotypes of 5 month-old transgenic mice. A: K5-uPA/uPAR, Plg+/+ mice developed alopecia and blisters (particularly prominent around the neck) whereas the fur of the two other genotypes is similar and comparable to wild-type mice. B and C: K5-uPA/uPAR, Plg+/+ mice are also characterized by the lack of whiskers, sparse tail hair, and a dull aspect of the tail, whereas Plg-deficient double-transgenic mice have full whiskers and a hairy and bright tail. In K5-uPA/uPAR, Plg+/− whiskers and tail hair have an intermediate density.

Figure 5

Hematoxylin and eosin-stained paraffin sections of adult back skin (A) and tail skin (B). A: Adult back skin epidermis thickness is largely increased in K5-uPA/uPAR, Plg+/+, slightly increased in K5-uPA/uPAR, Plg+/− and comparable to Plg−/− and wild-type adult back skin epidermis in K5-uPA/uPAR, Plg−/− mice. B: In K5-uPA/uPAR, Plg+/+ mice, tail epidermis is flattened and hair follicles are sparse. In K5-uPA/uPAR, Plg+/− tail skin, the epidermis is wavier and hair follicles are more numerous. In Plg-deficient double-transgenic mice, tail skin epidermis folding and the arrangement of hair follicle in clusters are similar to Plg−/− and wild-type tail skin. Bar: 100 μm in (A) and 200 μm in (B).