Uvomorulin mediates calcium-dependent aggregation of islet cells, whereas calcium-independent cell adhesion molecules distinguish between islet cell types.
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 Ca2+, 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/Ca2+ 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 Ca2+ was decreased by anti-uvomorulin Fab fragments from 67 +/- 4 to 25 +/- 3 (mean +/- SEM, n = 4, P less than 0.01). This level is not different from the degree of B-cell aggregation seen in the presence of 0.5 mM 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.
Laboratoires de Recherche Louis Jeantet, Centre Médical Universitaire, Geneva, Switzerland.
D G
DG
Rouiller
V
V
Cirulli
P A
PA
Halban
D000818:Animals; D015820:Cadherins / Q000502:physiology*; D002118:Calcium / Q000378:metabolism*; D015815:Cell Adhesion Molecules / Q000502:physiology*; D002449:Cell Aggregation; D002469:Cell Separation; D002478:Cells, Cultured; D005434:Flow Cytometry; D015151:Immunoblotting; D007140:Immunoglobulin Fab Fragments / Q000276:immunology; D007515:Islets of Langerhans / Q000166:cytology* / Q000378:metabolism; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains
D015820:Cadherins; D015815:Cell Adhesion Molecules; D007140:Immunoglobulin Fab Fragments; D002118:Calcium