First molecular characterization and immunolocalization of keratoepithelin in adult human skeletal muscle

Keratoepithelin (KE) is an extracellular matrix protein that binds collagens, fibronectin, decorin, biglycan and integrins, interconnecting extracellular matrix components with resident cells in several tissues. KE has a molecular mass of 68 kDa and harbours four FAS1 domains named after those identified in the insect cell adhesion molecule fasciclin I. In humans, KE is preferentially expressed by the corneal epithelial layer and liberated towards the corneal stroma but it was also detected in the lung and in the bladder smooth muscle. No detailed information is available on the distribution of this protein in other human tissues. In this work, we have raised a polyclonal antibody against the recombinantly expressed human fourth FAS1 domain which is able to specifically detect KE in human skeletal muscle tissue extracts. Immunofluorescence experiments indicate that KE is localized around the perimysium and endomysium of each skeletal muscle fiber. The same kind of analysis shows that in muscle sections from patients affected by different forms of muscular dystrophy KE is upregulated and widely distributed in fibrotic tissues. The muscle specific expression of KE was also demonstrated by RT-PCR. In human skeletal muscle, KE may help to build up a bridge between collagen VI and yet unidentified muscle receptor(s), adding to the complexity of the adhesive molecular network established between muscle fibers and the surrounding basement membrane.