Identification of sequence motifs that target neuronal nicotinic receptors to dendrites and axons

Neuronal nicotinic acetylcholine receptors (nAChRs) belong to a family of ligand-gated ion channels that play important roles in central and peripheral nervous systems. The subcellular distribution of neuronal nAChRs has important implications for function and is not well understood. Here, we analyzed the targeting of two major types of neuronal nAChRs by expressing epitope-tagged subunits in cultured hippocampal neurons. Surprisingly, the alpha 7 nAChR (alpha 7) and alpha 4/beta 2 nAChR (alpha 4 beta 2) displayed distinct patterns of expression, with alpha 7 targeted preferentially to the somatodendritic compartments, whereas alpha 4 beta 2 was localized to both axonal and dendritic domains. When fused to CD4 or IL2RA ( interleukin 2 receptor alpha subunit) proteins, which are normally distributed ubiquitously, the M3-M4 intracellular loop from the alpha 7 subunit promoted dendritic expression, whereas the homologous M3-M4 loop from the alpha 4 subunit led to surface axonal expression. Systemic screening and alanine substitution further identified a 25-residue leucine motif ([DE]XXXL[LI]) containing an axonal targeting sequence within the alpha 4 loop and a 48-residue dileucine and tyrosine motif (YXX circle divide) containing a dendritic targeting sequence from the alpha 7 loop. These results provide valuable information in understanding diverse roles of neuronal nAChRs in mediating and modulating synaptic transmission, synaptic plasticity, and nicotine addiction.