Expression and localization of actin and focal adhesion proteins in control and kindlin-1 null patient skin

Kindler syndrome (KS; OMIM173650) is an unusual, autosomal recessive skin disorder associated with trauma-induced blisters in early life followed by photosensitivity, poikiloderma, and an increased risk of malignancy. Defects in the actin/focal adhesion associated protein kindlin-1 (also known as kindlerin) encoded by the gene KIND1 have been shown to cause this disease. In human epidermis, kindlin-1 is expressed in epidermal keratinocytes, particularly within basal keratinocytes with an increase in staining at the dermal-epidermal junction. We have undertaken a detailed ultrastructural and immunohistochemical study in KS (n¼4) and control skin (n¼3) to examine morphology and the labeling of basement membrane, actin cytoskeletal, and focal contact-associated proteins. Transmission electron microscopy of KS skin showed disruption and reduplication of the lamina densa, together with sub-lamina separation. The number and structure of hemi-desmosomes and anchoring filaments appeared normal, although there was focal disruption in desmosome- and hemidesmosome-keratin filament attachment. This disruption in normal keratin filament assembly was most obvious at dermal splits and was associated with disorganized substratum bundles of actin filaments. Fluorescence microscopy showed increased epidermal expression of actin, alpha actinin, talin, vinculin, tenascin C, and RACK-1 in KS skin but no change in labeling with antibodies to filamin, tensin, focal adhesion kinase, paxillin, or tropomyosin. Immunostaining for protein kinase C was markedly reduced in basal keratinocytes in KS skin compared to controls. Immunogold electron microscopy using kindlin-1 antibody in control skin cytoplasm showed labeling over ends of microfilament-like structures. Taken together, our findings reveal a close spatial and functional relationship between kindlin-1, actin, some focal contact proteins and regulatory molecules that link the actin skeleton to the integrin extracellular matrix receptors. We hypothesize that the function of kindlin-1 might be to bind to the ends of actin microfilaments, linking focal adhesion proteins to integrin receptors at hemidesmosomes, thereby limiting the elongation of actin microfilaments. Conversely, a lack of kindlin-1 disrupts focal adhesion linkage and allows the unregulated proliferation of actin microfilaments, causing perturbations in the other cytoskeletal networks