SPECIFIC AGRIN ISOFORMS INDUCE CAMP RESPONSE ELEMENT-BINDING PROTEIN-PHOSPHORYLATION IN HIPPOCAMPAL-NEURONS

The synaptic basal lamina protein agrin is essential for the formation of neuromuscular junctions. Agrin mediates the postsynaptic clusterin g of acetylcholine receptors and regulates transcription in muscles. A grin expression is not restricted to motor neurons but can be demonstr ated throughout the CNS. The functional significance of agrin expressi on in neurons other than motor neurons is unknown. To test whether agr in triggers responses in neurons that lead to the activation of transc ription factors, we have analyzed phosphorylation of the transcription al regulatory site serine 133 of the transcription factor CREB (cAMP r esponse element binding protein) in primary hippocampal neurons. Our r esults indicate that the neuronal (Ag4,8), but not the non-neuronal (A g0,0), isoform of agrin-induces CREB phosphorylation in hippocampal ne urons. The kinetics of agrin- and BDNF-induced CREB phosphorylation ar e similar: peak levels are reached in minutes and are strongly reduced 2 hr later. Neuronal responses to agrin require extracellular calcium , and, in contrast to tyrosine kinase inhibitors, the specific inhibit ion of protein kinase A (PKA) does not affect agrin-evoked CREB phosph orylation. Our results show that hippocampal neurons specifically resp ond to neuronal agrin in a Ca2+-dependent manner and via the activatio n of tyrosine kinases.