Mouse -Opioid Receptor mRNA Differential Transport in Neurons

Three -opioid receptor (KOR) mRNA isoforms have been detected in different parts of the central nervous system. At the cellular level, three KOR mRNA isoforms are also differentially distributed in the axons and cell bodies of adult mouse trigeminal neurons, as well as in the processes and cell bodies of differentiated P19 neurons. To determine the molecular basis underlying differential distribution of KOR mRNA isoforms, a GFP-fused RNA binding domain, MS2, was generated and used to trace movement of KOR mRNA tagged with the MS2binding sequence in living neurons of dorsal root ganglia and in differentiated P19 neurons. The 5 and 3 -untranslated regions (UTRs) of KOR, either alone or in combination, are able to mediate transport of mRNAs to processes of P19 neurons and axons of dorsal root ganglia. The efficiency of mRNA transport mediated by each 5 -UTR of KOR varies among the three isoforms; isoform A is most efficient. This study demonstrates the biological activity of the UTRs of KOR mRNA isoforms in directing differential transport of mRNA in mammalian neurons. Opioid receptors interact with opiate drugs and endogenous opioid ligands to affect pain sensation, consciousness, and autonomic functions. Three major opioid receptor types, , , and , have been defined (Goldstein and Naidu, 1989; Masabumi and Satoh, 1995) that belong to the super family of G-protein-coupled membrane receptors (Wei and Loh, 2002). For each gene, mRNA isoforms have been detected but only one type of protein is produced. However, pharmacological studies indicate existence of more than one subtype of each receptor (Goldstein and Naidu, 1989; Masabumi and Satoh, 1995), which remains to be elucidated at the molecular level. -Opioid receptor (KOR) proteins were detected both preand postsynaptically (Drak et al., 1996; Shuster et al., 1999). Activation of presynaptic KOR proteins, together with muscarinic receptors, inhibited calcium-dependent glutamate release (Rawls et al., 1999). The mouse KOR gene produces three mRNA isoforms varied at 5 -untranslated regions (5 UTRs). We have previously demonstrated differential distribution of these KOR mRNA isoforms in the nervous systems and retinoic acid-induced, differentiated neurons of P19, as well as variation in the stability and translation efficiency of these KOR mRNA isoforms (Wei et al., 2000; Bi et al., 2001). However, how the different isoforms of KOR mRNAs can be present in different parts of brain areas and in vitro differentiated neurons remains unknown. In particular, whether these alternatively spliced KOR mRNAs are differentially distributed in cell bodies and fibers, including axons, is un-