Identification of COL7A1 Alternative Splicing Inserting 9 Amino Acid Residues Into the Fibronectin Type III Linker Domain

Type VII collagen is the major component of anchoring fibrils within the cutaneous basement membrane zone. The large amino-terminal noncollagenous domain of type VII collagen interacts with various extracellular matrix proteins and contributes to the dermal–epidermal attachment. The purpose of this study was to detect alternative splicing of COL7A1 transcript encoding the noncollagenous 1 domain. The alternative splicing in this region may affect interactions of the noncollagenous 1 domain with extracellular matrix proteins and also dermal–epidermal adhesion. Thus we examined expression of the alternative splicing in situations relating to wound healing and skin remodeling that required dermal–epidermal binding and detachment. Amplification of overlapping cDNA from keratinocytes using reverse transcription–polymerase chain reaction identified alternative splicing, which was generated by a different exon 18 acceptor site 27 bp upstream from the common acceptor site. Expression of this alternatively spliced transcript differed among several cell types. The nine amino acid residues GPLTLPLSP from the 27 bp nucleotides were inserted into the linker of fibronectin type III domains. This insertion was suggested to contribute to flexibility of the linker of fibronectin type III domains and may affect the interactions between the noncollagenous 1 domain and extracellular matrix proteins. Treatment with transforming growth factor-β 1, which is known to promote wound healing and skin remodeling, enhanced the expression of this 27 bp transcript. Furthermore, keratinocyte biopsies from the wound edge of patients with epithelizing skin ulcers showed a significant increase in the 27 bp transcript expression compared with normal keratinocytes from steady-state body sites. These results suggest that amino acid variation of this alternative splicing may have some role in dermal–epidermal adhesion, wound healing, and skin remodeling. To the best of our knowledge, this is the first evidence of alternative splice insertion of a small peptide into the linker region of the fibronectin type III domains, a common motif within modular proteins