We have recently studied the synthesis and deposition of the extracellular matrix (ECM) proteins laminin (LM), fibronectin (FN) and type IV collagen by human neuroblastoma (NB) cells in vitro, before and after differentiation with dibutyrylcyclic (dbc) AMP and retinoic acid (RA). The synthesis of all the above ECM proteins was confirmed by polyacrylamide gel electrophoresis (PAGE) of the conditioned culture media of several NB cell lines, after metabolic radiolabelling with 3H-leucine and 3H-proline. Deposition of the proteins on the cell layers was confirmed by immunofluorescence (IF), using antisera against LM, FN and type IV collagen on the cell layers. The pattern of expression of the above proteins by specific morphologic phenotypes suggested to us that these proteins may play a role in neuronal-Schwann cell interaction and hence differentiation of NB. Specifically, the fact that LM was only present on the surface and the cytoplasm of flat cells with Schwannian characteristics and absent from neuronal cells, led us to believe that: (1) cells with features of Schwann cells and neurones were appearing in culture and (2) these cells might recapitulate normal Schwann cell neurone interaction. In this case, the abundant cell surface LM on Schwann cells might be a chemoattractant for LM-poor, LM-receptor-rich neuronal cells. The existence of such a receptor to laminin has been recently confirmed by several investigators for breast carcinoma, fibrosarcoma and muscle cells. Moreover, others have shown that neurons extend neurites on laminin substrata. To evaluate the validity of this hypothesis, (1) we have searched for the presence of a laminin receptor in NB cell lines, its exact localization in specific cell phenotypes, before and after neuronal and/or Schwannian differentiation using a monoclonal antibody against the LM receptor and (2) we have searched for possible differences in the expression of the mRNA which is involved in the synthesis of the LM receptor.
