| Cytokeratin 3 (CK3, gene KRT3) is a type II intermediate filament keratin that forms an obligate heteropolymeric pair with the type I keratin K12 to build the cornea-specific cytoskeletal scaffold in differentiated corneal epithelium. This network supports mechanical resilience, barrier function, and the maintenance of corneal transparency by anchoring to desmosomal and hemidesmosomal complexes and integrating with the epithelial basement membrane. CK3/K12 filaments assemble into a dense cytoplasmic lattice whose organization depends on highly conserved helix boundary motifs and the glycine-rich tail domain—regions that mediate longitudinal filament assembly and lateral filament–filament packing. Even single amino-acid substitutions in these motifs can alter filament stability and increase susceptibility to mechanical shear. Within the corneal epithelium, this keratin pair defines the differentiated suprabasal compartment, marking the transition from limbal progenitors to mature central epithelial cells and coupling cytoskeletal status to junctional complexes that stabilize epithelial sheets and preserve an optically smooth surface. Pathogenic missense mutations in KRT3, particularly those affecting the helix boundary and glycine-loop regions, disrupt heteropolymer assembly with K12, generate keratin aggregates, and weaken the intermediate filament network. This results in epithelial cell fragility, microcyst formation, and recurrent erosions characteristic of Meesmann corneal dystrophy, demonstrating how precise CK3 structure–function relationships can translate into tissue-level mechanical failure. Genetic linkage and sequencing studies show that disease-associated KRT3 variants segregate with Meesmann corneal dystrophy in affected families and are absent from unaffected controls. Different mutations in either KRT3 or KRT12 can produce a similar phenotype by compromising the same filament-stabilizing motifs, reinforcing the concept that corneal epithelial integrity depends on balanced expression and correct pairing of this keratin duo. CK3 also functions as a corneal epithelial lineage marker. Immunohistochemical detection of CK3/CK12 defines cornea-type differentiation and reveals altered expression or mislocalization of these keratins in certain corneal dystrophies. Changes in CK3 levels after chemical injury or epithelial loss correlate with shedding and re-epithelialization dynamics, linking its expression pattern to injury responses and epithelial renewal. |
