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. Durin g the past years, mRNA localization to dendrites of nerve cells has been ch aracterized in detail. The functional role of these transcripts appears to be obvious: Since dendrites are equipped with the basic translational machi nery, certain proteins are likely to be synthesized on-site. Targeting of m RNAs 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 transl ation is still preliminary. In most differentiated mammalian neurons the ax on 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 elusi ve until today. Recent evidence suggests that mRNA transport to neurites in cluding the future axon and local translation might play a role in nerve ce ll maturation. mRNA targeting to the axon of young neurons is strictly corr elated with differentiation. It is no longer observed in fully matured neur ons. Finally, for many years invertebrate neurons have served as model syst ems to investigate axonal mRNA transport and its physiological relevance. T here is no doubt that protein synthesis does take place in the axonal domai n. However, it has to be considered that invertebrate neurons develop only one type of neurite, referred to as the axon. These axons are different whe n compared with those of vertebrate nerve cells since they combine characte ristics of dendrites and axons. In fact, current evidence supports the view that the axonal mRNAs in invertebrate nerve cells have functions comparabl e to those of transcripts residing in the dendrites of vertebrate neurons.