Normal brain function depends on the rapid transfer of information between neurons at highly specialized structures known as synapses. Understanding the mechanisms that direct the formation of synapses during development and their maintenance in the adult are, therefore, fundamental problems in neurobiology. Most of what we know about the development of chemical synapses comes from studies of the neuromuscular junction, where signals exchanged between motor neurons and the skeletal muscle fibers they innervate coordinate differentiation of the motor nerve terminal and postsynaptic apparatus in the muscle fiber. Compelling evidence now exists to show that one such signal is an extracellular matrix molecule called agrin, which plays a key role in the motor neuron induced organization of synaptic components in the muscle fiber. In contrast to the neuromuscular junction, relatively little is known about the mechanisms that direct neuron-neuron synapse formation in the central nervous system. We have shown that agrin is expressed in mammalian brain, consistent with a role for agrin in neuron-neuron synapse formation. Surprisingly, early stages of synaptogenesis between cortical neurons cultured from agrin knock-out mice appear normal, raising the question of what, if any, function agrin serves in brain. We have addressed this issue using expression of the immediate early gene c-fos and neural injury to monitor neuronal activity and show that agrin-deficient cultured cortical neurons exhibit reduced sensitivity to excitatory amino acids. These studies are the first to establish a CNS neuronal phenotype for a mutation in the agrin gene and suggest an important role for agrin in regulating glutamate receptor expression or function. Experiments are proposed that seek to test this hypothesis and identify the cellular defect caused by the mutation. Evidence that agrin is required for normal development of cortical neurons predicts the existence of a neuronal signal transduction pathway for agrin. Preliminary data confirm this hypothesis showing that agrin induces a dose-dependent saturable increase in Fos levels in cultured cortical neurons. Biochemical studies indicate that agrin induction of c-fos is similar to but distinct from signals mediating agrin-induced clustering of acetylcholine receptors in muscle. Additional experiments are proposed to explore these observations in more detail, culminating with cloning the receptor for agrin that regulates the neuronal signal transduction pathway. The results of these studies will contribute to our understanding of neural development and synapse formation in the brain. This knowledge will be directly relevant to the management of brain disorders characterized by cognitive deterioration such as Alzheimer disease, as well as regeneration following traumatic injury.
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