Sequential onset of presynaptic molecules during olfactory sensory neuron maturation.

Differentiated olfactory sensory neurons express specific presynaptic proteins, including enzymes involved in neurotransmitter transport and proteins involved in the trafficking and release of synaptic vesicles. Studying the regulation of these presynaptic proteins will help to elucidate the presynaptic differentiation process that ultimately leads to synapse formation. It has been postulated that the formation of a synapse between the axons of the sensory neurons and the dendrites of second order neurons in the olfactory bulb is a critical step in the processes of sensory neuron maturation. One approach to study the relationship between synaptogenesis and sensory neuron maturation is to examine the expression patterns of synaptic molecules through the olfactory neuron lineage. To this end we designed specific in situ hybridization probes to target messengers for proteins involved in presynaptic vesicle release. Our findings show that, as they mature, mouse olfactory neurons sequentially express specific presynaptic genes. Furthermore, the different patterns of expression of these presynaptic genes suggest the existence of discrete steps in presynaptic development: genes encoding proteins involved in scaffolding show an early onset of expression, whereas expression of genes encoding proteins involved in the regulation of vesicle release starts later. In particular, the signature molecule for glutamatergic neurons vesicle glutamate transporter 2 shows the latest onset of expression. In addition, contact with the targets in the olfactory bulb is not controlling presynaptic protein gene expression, suggesting that olfactory sensory neurons follow an intrinsic program of development.