Uvomorulin mediates calcium-dependent aggregation of islet cells, whereas calcium-independent cell adhesion molecules distinguish between islet cell types

Abstract

Rat islets of Langerhans are organized as a core of B-cells surrounded by non-B-cells. It is believed that cell type segregation during histogenesis is the result of the differential expression of cell adhesion molecules (CAMs). Since we have previously shown that in contrast to non-B-cells, homotypic adhesion of pancreatic B-cells is dependent on the presence of Ca 2+, the possibility exists that Ca 2+-dependent CAMs (cadherins) might be in part responsible for islet topography. We now demonstrate that after selective removal of Ca 2+-independent CAMs from the surface of islet cells by mild trypsin Ca 2+ digestion (TC-treatment), there is no significant difference in homotypic adhesion between sorted B- and non-B-cells in the presence of calcium, suggesting an identical deployment of cadherins. Flow cytometric analysis reveals high levels of uvomorulin on both B- and non-B-cells, without any difference between the two populations. On a “1 to 100” scale, B-cell aggregation in the presence of Ca 2+ was decreased by anti-uvomorulin Fab fragments from 67 ± 4 to 25 ± 3 (mean ± SEM, n = 4, P < 0.01). This level is not different from the degree of B-cell aggregation seen in the presence of 0.5 m M EDTA (22 ± 2). Aggregation of non-B-cells was only slightly decreased by anti-uvomorulin Fab fragments (from 69 ± 3 to 52 ± 4). However, after TC-treatment, homotypic cell aggregation of both B- and non-B-cells was completely inhibited by anti-uvomorulin Fab fragments. Thus, uvomorulin appears to be the only functional cadherin on islet cells, and cell type aggregation properties diverge only by virtue of higher levels of Ca 2+-independent CAMs on non-B-cells. Fab fragments with the property of perturbing islet cell aggregation in the absence but not in the presence of calcium also prevented pseudoislet organization in vitro, suggesting that Ca 2+-independent CAMs play the major role in islet cell type segregation. In conclusion, the results show that uvomorulin is responsible for the Ca 2+-dependent aggregation of islet cells and suggest that the cellular organization within islets or pseudoislets results from the different level of Ca 2+-independent CAMs on islet cell types.