Expression of Functionally Distinct Variants of the β4A Integrin Subunit in Relation to the Differentiation State in Human Intestinal Cells

Abstract

Integrins are important mediators of cell-laminin interactions. In the small intestinal epithelium, which consists of spatially separated proliferative and differentiated cell populations located, respectively, in the crypt and on the villus, laminins and laminin-binding integrins are differentially expressed along the cryptvillus axis. One exception to this is the integrin ␣ 6 ␤ 4 , which is thought to be ubiquitously expressed by intestinal cells. However, in this study, a re-evaluation of the ␤ 4 subunit expression with different antibodies revealed that two forms of ␤ 4 exist in the human intestinal epithelium. Furthermore, we show that differentiated enterocytes express a full-length 205-kDa ␤ 4 A subunit, whereas undifferentiated crypt cells express a novel ␤ 4 A subunit that does not contain the COOH-terminal segment of the cytoplasmic domain (␤ 4 A ctd؊ ). This new form was not found to arise from alternative ␤ 4 mRNA splicing. Moreover, we found that these two ␤ 4 A forms can associate into ␣ 6 ␤ 4 A complexes; however, the ␤ 4 A ctd؊ integrin expressed by the undifferentiated crypt cells is not functional for adhesion to laminin-5. Hence, these studies identify a novel ␣ 6 ␤ 4 A ctd؊ integrin expressed in undifferentiated intestinal crypt cells that is functionally distinct.

Differential expression of the integrins α6Aβ4 and α6Bβ4 along the crypt–villus axis in the human small intestine

“…3b; statistically signiWcantly diVerent from SC, P < 0.01, Tukey's One Way Analysis of Variance (ANOVA), n = 3]. The Wndings that the two splice variants of the 6 integrin are diVerentially expressed in the proliferative and diVerentiated compartments of the gut, and that there is an association between the ratio of these two splice variants with the stage of diVerentiation, are consistent with previous Wndings of distinct expression of components of the integrin-ECM system in gastrointestinal biology (Basora et al 1997(Basora et al , 1999 supporting the importance of interactions between the epithelium of the intestinal tract and the underlying BM (Beaulieu 1997;Teller and Beaulieu 2001).…”

“…As shown previously for the 4 subunit (Basora et al 1999), immunodetection of the 6 integrin subunit using an antibody directed against the extracellular domain (G0H3) (Sonnenberg et al 1987) yielded ubiquitous staining at the base of the epithelial cells in both villus and crypt (Fig. 1a).…”

“…We have previously demonstrated that the 6 4 integrin is the only 6 containing integrin in the human intestine (Basora et al 1999). In this context, it is noteworthy that there is substantial evidence for the diVerential capacity of the 6A 1 and 6B 1 integrins to initiate intracellular signaling Wei et al 1998) and facilitate migration on laminin , but to our knowledge, no study has ever demonstrated a functional diVerence between the 6A 4 and 6B 4 integrins, making the present work the Wrst to demonstrate such a diVerence.…”

“…The crypt-like human intestinal epithelial HIEC cells and Caco-2/15 cells were grown as described previously (Basora et al 1999;Perreault and Beaulieu 1996;Vachon and Beaulieu 1995).…”

“…In the human intestine, the 6 subunit dimerizes with the 4 subunit forming the 6 4 integrin (Basora et al 1999). The relatively simple structural and functional renewal unit of the small intestine, the crypt-villus axis, makes it an attractive model for the study of epithelial cell proliferation and maturation (Babyatsky and Podolsky 1995).…”