Regulation of agrin expression in hippocampal neurons by cell contact and electrical activity.
Most synapses contain high concentrations of neurotransmitter receptors in the postsynaptic plasma membrane. Agrin (Ag) is an extracellular matrix protein necessary for the localization of acetylcholine receptors at the neuromuscular junction and for the differentiation of synapses in hippocampal neurons in vitro. The temporal pattern of agrin expression during the development of the central nervous system (CNS) is consistent with the notion that agrin expression is regulated during synaptogenesis. To identify the processes underlying this regulation, we have analyzed levels and alternative splicing of agrin mRNA in primary hippocampal neurons. Our results indicate that in the initial phases of synapse formation, contact-mediated processes and action potential-dependent neurotransmission regulate agrin mRNA expression, while secreted factors from glial cells, but not from hippocampal neurons, influence the alternative splicing of agrin mRNA. Previous studies have shown that specific agrin isoforms are able to induce the activation of a transcription factor and that secreted agrin associates with cellular surfaces. Therefore, we have tested whether agrin isoforms contribute to the contact-mediated induction of agrin expression in hippocampal neurons. None of the agrin isoforms tested in this study revealed this activity. Finally, we show that the role of evoked neural transmission in controlling agrin transcription changes during differentiation in vitro.