Axonal mRNAs: functional significance in vertebrates and invertebrates.

Sorting of defined mRNA species to distinct cytoplasmic regions is observed throughout the animal kingdom in many cell types, including neurons. During the past years, mRNA localization to dendrites of nerve cells has been characterized in detail. The functional role of these transcripts appears to be obvious: Since dendrites are equipped with the basic translational machinery, certain proteins are likely to be synthesized on-site. Targeting of mRNAs to the axon of vertebrate neurons is less well understood. Even though some vertebrate nerve cells such as goldfish Mauthner neurons seem to have ribosomes within the axonal compartment, evidence for ongoing local translation is still preliminary. In most differentiated mammalian neurons the axon is thought to lack mRNAs and a protein synthesizing machinery. Although a few nerve cell types harbour substantial amounts of distinct mRNA species within the axonal domain, their functional significance has remained elusive until today. Recent evidence suggests that mRNA transport to neurites including the future axon and local translation might play a role in nerve cell maturation. mRNA targeting to the axon of young neurons is strictly correlated with differentiation. It is no longer observed in fully matured neurons. Finally, for many years invertebrate neurons have served as model systems to investigate axonal mRNA transport and its physiological relevance. There is no doubt that protein synthesis does take place in the axonal domain. However, it has to be considered that invertebrate neurons develop only one type of neurite, referred to as the axon. These axons are different when compared with those of vertebrate nerve cells since they combine characteristics of dendrites and axons. In fact, current evidence supports the view that the axonal mRNAs in invertebrate nerve cells have functions comparable to those of transcripts residing in the dendrites of vertebrate neurons.