Delayed Degradation and Impaired Dendritic Delivery of Intron-Lacking -/ mRNA in Transgenic Mice.

is a unique immediate early gene (IEG) whose expression is induced as synapses are modified during learning. Newly-synthesized mRNA is rapidly transported throughout dendrites and localizes near recently activated synapses. mRNA levels are regulated by rapid degradation, which is accelerated by synaptic activity in a translation-dependent process. One possible mechanism is nonsense-mediated mRNA decay (NMD), which depends on the presence of a splice junction in the 3'UTR. Here, we test this hypothesis using transgenic mice that express . Because the transgene was constructed from cDNA, it lacks intron structures in the 3'UTR that are present in the endogenous gene. NMD depends on the presence of proteins of the exon junction complex (EJC) downstream of a stop codon, so should not undergo NMD. Assessment of mRNA rundown in the presence of the transcription inhibitor actinomycin-D confirmed delayed degradation of mRNA. mRNA and protein are expressed at much higher levels in transgenic mice under basal and activated conditions but mRNA does not enter dendrites efficiently. In a physiological assay in which cycloheximide (CHX) was infused after induction of by seizures, there were increases in endogenous mRNA levels consistent with translation-dependent mRNA decay but this was not seen with mRNA. Taken together, our results indicate: (1) mRNA degradation occurs via a mechanism with characteristics of NMD; (2) rapid dendritic delivery of newly synthesized mRNA after induction may depend in part on prior splicing of the 3'UTR.