Peptide localization and gene structure of cryptdin 4, a differentially expressed mouse paneth cell alpha-defensin

Paneth cells in crypts of the small intestine express antimicrobial peptide s, including ol defensins, termed cryptdins in mice. Of the known Paneth ce ll alpha-defensins, the cryptdin 4 gene is unique, because it is inactive i n the duodenum and expressed at maximal levels in the distal small bowel (D . Darmoul and A. J. Ouellette, Am. J, Physiol, 271:G68-G74, 1996). With a c ryptdin 4-specific antibody, immunohistochemical staining of ileal Paneth c ells was strong and specific for cytoplasmic granules, demonstrating that t his microbicidal peptide is a secretory product of Paneth cells in the dist al small intestine. Consistent with the pattern of cryptdin 4 mRNA distribu tion along the length of the gut, the cryptdin 4 peptide was not detected i n duodenum. Structurally, the cryptdin 4 gene resembles other Paneth cell a lpha-defensin genes. Its two exons, transcriptional start site, intron, spl ice sites, and 3' flanking sequences are characteristic of the highly conse rved mouse alpha-defensin genes. However, in the region upstream of the tra nscriptional initiation site, the cryptdin 4 gene contains a repeated 130-b p element that is unique to this alpha-defensin gene. Every independent cry ptdin 4 genomic clone examined carries the repeated element, which contains putative recognition sequences for TF-IID-EIIA, cMyc-RS-1, and IgHC.2/CuE1 .1; the repeat proximal to the start of transcription replaces DNA at the c orresponding position in other mouse alpha-defensin genes. We speculate tha t this unique duplicated element may have a cis-acting regulatory role in t he positional specificity of cryptdin 4 gene expression,