What Makes An Axon An Axon?

[Guest guest]

What Makes An Axon An Axon?

http://medicalnewscenter.com/out/out.cgi?

http://www.sciencedaily.com/releases/2008/11/081110112055.htm

Inside every axon is a dendrite waiting to get out. Hedstrom et al.

converted mature axons into dendrites by banishing a protein crucial

for neuron development. The results suggest that this transformation

could occur after nerve cell damage.

At the junction between the axon and the cell body is the axon

initial segment (AIS), which sparks action potentials. Researchers

also suspect that the AIS enables an axon to maintain its identity.

Although scientists have teased out some of the molecular events that

determine whether a neuron outgrowth will become an axon or a

dendrite, they know little about what keeps these structures distinct.

In cultured neurons, Hedstrom et al. used RNAi to cut production of

ankyrinG, a protein that helps spur formation of the AIS. Loss of

ankyrinG caused the AIS to disappear. For example, sodium channels

that normally crowd the AIS's plasma membrane dispersed. When

ankyrinG was absent the erstwhile axons began to resemble dendrites,

sprouting spines and developing excitatory synapses. Characteristic

molecules from the cell body and dendrites, such as the microtubule

polymerizing protein MAP2, also infiltrated the axons.

The AIS appears to serve as a filter that screens out dendritic

proteins, Hedstrom et al. conclude. How the structure bars some

proteins from the axon is still uncertain. Injuries and diseases can

transform dendrites into axons. The findings raise the possibility

that these insults could cause the reverse transformation by changing

the amount of ankyrinG and altering the AIS.

Journal reference: Hedstrom, K.L., et al. 2008. J. Cell Biol. Nov.

17, 2008 doi:10.1083/jcb.200806112.