The molecules which control the patterns of cell division, growth, and precise interconnections characteristic of the central nervous system still remain largely unidentified. The protein platelet-derived growth factor (PDGF) has been shown to mediate interactions among glial cells in vitro. More recent evidence has indicated that PDGF may also be involved in controlling communication between neurons and glial cells and among neurons. The presence of receptors for PDGF on neurons of the developing nervous system is an essential piece of evidence in this chain of events. Ganglion cells are labeled with antibodies to PDGF receptor only during the period of active process outgrowth. These findings suggest that PDGF is used as a mediator of intercellular signaling during neuronal development.

After infection with sporozoites of the protozoon Theileria parva (Tp) bovine T cells are readily transformed to permanent growth in vivo and in vitro. Their transformed state depends on the constant presence of the parasite but membrane signals remain important. Non-receptor tyrosine kinases play a critical role in the transduction of membrane signals in haematopoietic cells. We have investigated Src-family kinases in bovine T cells transformed by Tp. The T cell receptor-associated tyrosine kinase p60fyn had high activity in all cell lines tested. In addition, weak phosphorylation of 2 novel bands was observed associated with Fyn. In contrast to Fyn, enzymatic activity of p56lck, which in T cells has an essential role in signalling, was low. Furthermore, 1 of 3 Tp transformed cell lines was completely devoid of p56lck indicating that the enzyme is not necessary for the Tp dependent growth of the T cells. In addition to p60fyn and p56lck weak enzymatic activity of 1 splice variant of p53/56lyn was observed after infection of T cells with Tp. These data show that growth transformation by Tp influences kinase activity in bovine T cells. However, they also prove that p56lck does not play an essential role in the transformation mechanism.