The human brain consists of about 100 billion (1011) neurons, which altogether form about 100 trillion (1014) synaptic connections with each other. A crucial mechanism for the generation of this complex wiring pattern is the formation of neuronal branches. The neurobiologists Dr. Hannes Schmidt and Professor Fritz G. Rathjen at the Max Delbrck Center for Molecular Medicine (MDC) Berlin-Buch, Germany, have now discovered a molecule that regulates this vital process. At the same time they have succeeded in elucidating the signaling cascade induced by this molecule (PNAS, Early Edition, 2009, doi:10.1073)*.
Through the ramification of its fiber-like axon, a single neuron can send branches and thus transmit information into several target areas at the same time. In principle, neurobiologists distinguish between two kinds of axonal branching: branching of the growth cone at the tip of an axon and the sprouting of collaterals (interstitial branching) from the axon shaft.
Both forms of axonal branching can be observed in sensory neurons, which transmit the sensation of touch, pain and temperature, among others. When the axons of these neurons reach the spinal cord, their growth cones first split (bifurcate) and consequently the axons divide into two branches growing in opposite directions. Later new branches sprout from the shaft of these daughter axons which penetrate the gray matter of the spinal cord.
Through investigations on sensory neurons, Dr. Hannes Schmidt and his colleagues were able to identify a protein which triggers the splitting of the growth cone of the sensory axons: the peptide CNP (the abbreviation stands for C-type natriuretic peptide). In transgenic mice the scientists were able to show that CNP is formed in the spinal cord precisely when sensory neurons grow into it. In the absence of CNP bifurcation can no longer occur which results in reduced neuronal connectivity in the spinal cord.
The new findings suppl
|Contact: Barbara Bachtler|
Helmholtz Association of German Research Centres