Nerve cells with stoppage: Evidence for a molecular key step in the process of signal transduction between nerve cells

Munc13-1 regulates the readily releasable pool of synaptic vesicles. Synaptic vesicles are the key organelles in neurotransmitter release. At the synapse, they cycle between an early endosomal compartment and the synaptic plasma membrane. A large number of proteins regulates the various fusion, fission and transport reactions that constitute the life cycle of synaptic vesicles. Vesicles are generated by budding from early endosomes and are loaded with neurotransmitter (NT). After a translocation process, they dock at the active zone and mature to a fusion competent state (red). The number of fusion competent or primed vesicles defines the readily releasable vesicle pool and determines the functionality of the synapse. In response to activation and a concomitant rise in the intracellular calcium concentration, only fusion competent/primed vesicles fuse with the plasma membrane and release their content. Vesicular protein and lipid components are then retrieved by endocytosis and recycled via early endosomes. Munc13-1 mediates vesicle priming to fusion competence. In nerve cells lacking Munc13-1, the vesicle cycle as depicted here is arrested at the priming step, leading to an almost complete loss of fusion competent vesicles and a functional shutdown of synapses. Full size image available through contact

By eliminating a single synaptic protein, German and US-american researchers provide the first direct evidence that vesicles, which act as cellular transporters, underlie a maturation process making them competent for membrane fusion in the context of neurotransmitter release at nerve cell synapses (Nature, 29 July 1999).

Information in the brain is transmitted at synapses which are specialized contact zones between signal-sending and signal-receiving nerve cells. When stimulated, a sending nerve cell generates an electric signal (the action potential) that travels along a spezialized cell process (the axon) to the synapse. There, the arriving signal triggers the release of neurotransmitter molecules, which now diffuse to the signal-receiving nerve cell, and change its physiological state.

A large number of proteins is involved in this complex cellular process. They regulate the turnover of the small cellular transporters known as vesicles, which first store the neurotransmitter molecules and then upon stimulation release them by fusing with the synaptic cell membrane (Figure 1). Under resting conditions nerve cells remain responsive because each of their synapses maintains a pool of transmitter-filled vesicles which are competent for membrane fusion and therefore can release neurotransmitter immediatly in response to an arriving action potential. To get fusion competent the vesicles run a maturation reaction which is analogous to the cocking of a gun, and represents a key step in the process of neurotransmitter release. Vesicle maturation has been inferred from numerous experimental observations but up to now was never demonstrated directly.

Neurons lacking Munc13-1 show drastically reduced synaptic transmission (top). This is due to an almost complete absence of readily releasable synaptic vesicles that can be assayed by applying hypertonic sucrose solution (bottom). Full size image available through contact

Iris Augustin in the laboratory of Nils Brose at the Max Planck Institute for Experimental Medicine in Goettingen/Germany generated mutant mice lacking a single synaptic protein, Munc13-1. In close collaboration with the electrophysiologist Christian Rosenmund of the Max Planck Institute for Biophysical Chemistry in Goettingen/Germany and with Thomas C. Sudhof of the UT Southwestern Medical Center in Dallas/Texas, the scientists found that mice lacking Munc13-1 die immediately after birth because their nerve cells have no readily releasable vesicles (Figure 2). Combining electrophysiological and structural analyses, the group demonstrates that synapses lacking Munc13-1 are arrested at the vesicle maturation or 'cocking' state, leading to a complete shutdown of synaptic transmission. Interestingly, this role of Munc13-1 is specific for synapses using the excitatory neurotransmitter glutamate. Other synapses appear to function independently of Munc13-1, indicating the existence of transmitter-specific vesicle maturation processes.

"The findings provide the first direct evidence for a vesicle maturation or 'cocking' reaction in the synaptic vesicle cycle and show that Munc13-1 is an essential molecular player in this process", says Brose. "We show that vesicle maturation is of prime importance in nerve cells. When it is abolished in Munc13-1-deficient mice, nerve cells are unable to signal, just like a loaded revolver can not fire if it is not cocked."

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