Anthony G. Lau

Distinct 3'UTRs differentially regulate activity-dependent translation of brain-derived neurotrophic factor (BDNF)

Expression of the brain-derived neurotrophic factor (BDNF) is under tight regulation to accommodate its intricate roles in controlling brain function. Transcription of BDNF initiates from multiple promoters in response to distinct stimulation cues. However, regardless which promoter is used, all BDNF transcripts are processed at two alternative polyadenylation sites, generating two pools of mRNAs that carry either a long or a short 3′UTR, both encoding the same BDNF protein. Whether and how the two distinct 3′UTRs may differentially regulate BDNF translation in response to neuronal activity changes is an intriguing and challenging question. We report here that the long BDNF 3′UTR is a bona fide cis-acting translation suppressor at rest whereas the short 3′UTR mediates active translation to maintain basal levels of BDNF protein production. Upon neuronal activation, the long BDNF 3′UTR, but not the short 3′UTR, imparts rapid and robust activation of translation from a reporter. Importantly, the endogenous long 3′UTR BDNF mRNA specifically undergoes markedly enhanced polyribosome association in the hippocampus in response to pilocarpine induced-seizure before transcriptional up-regulation of BDNF. Furthermore, BDNF protein level is quickly increased in the hippocampus upon seizure-induced neuronal activation, accompanied by a robust activation of the tropomyosin-related receptor tyrosine kinase B. These observations reveal a mechanism for activity-dependent control of BDNF translation and tropomyosin-related receptor tyrosine kinase B signaling in brain neurons.

Glycosylation of β1-adrenergic receptors regulates receptor surface expression and dimerization

The beta(1)-adrenergic receptor (beta(1)AR) has one predicted site of N-linked glycosylation on its extracellular amino-terminus, but the glycosylation and potential functional importance of this site have not yet been examined. We show here that the beta(1)AR is glycosylated in various cell types and that mutation of the single predicted site of N-linked glycosylation (N15A) results in the formation of receptors that are not N-glycosylated. The beta(1)AR N15A mutant exhibited significantly decreased basal surface expression relative to the wild-type receptor but had no detectable deficits in ligand binding or agonist-promoted internalization. Co-immunoprecipitation experiments using Flag-tagged and HA-tagged receptors demonstrated that the beta(1)AR-N15A mutant receptor exhibits a markedly reduced capacity for dimerization relative to wild-type beta(1)AR. These data reveal that the beta(1)AR is glycosylated on Asn15 and that this glycosylation plays a role in regulating beta(1)AR surface expression and dimerization.

Translational regulation of GluR2 mRNAs in rat hippocampus by alternative 3′ untranslated regions

The glutamate receptor 2 (GluR2) subunit determines many of the functional properties of the α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate subtype of glutamate receptor. The roles of untranslated regions (UTRs) in mRNA stability, transport, or translation are increasingly recognized. The 3′ end of the GluR2 transcripts are alternatively processed to form a short and long 3′UTR, giving rise to two pools of GluR2 mRNA of 4 and 6 kb in length, respectively, in the mammalian brain. However, the role of these alternative 3′UTRs in GluR2 expression has not been reported. We demonstrate that in the cytoplasm of rat hippocampus, native GluR2 mRNAs bearing the long 3′UTR are mostly retained in translationally dormant complexes of ribosome‐free messenger ribonucleoprotein (mRNP), whereas GluR2 transcripts bearing the short 3′UTR are predominantly associated with actively translating ribosomes. One day after pilocarpine‐induced status epilepticus (SE), the levels of both long and short GluR2 transcripts were markedly decreased in rat hippocampus. However, GluR2 mRNAs bearing the long 3′‐UTRs were shifted from untranslating mRNP complexes to ribosome‐containing complexes after SE, pointing to a selective translational derepression of GluR2 mRNA mediated by the long 3′UTR. In Xenopus oocytes, expression of firefly luciferase reporters bearing alternative GluR2 3′UTRs confirmed that the long 3′UTR is sufficient to suppress translation. The stability of reporter mRNAs in oocytes was not significantly influenced by alternative 5′ or 3′UTRs of GluR2 over the time period examined. Overall, our findings that the long 3′UTR of GluR2 mRNA alone is sufficient to suppress translation, and the evidence for seizure‐induced derepression of translation of GluR2 via the long 3′UTR strongly suggests that a regulatory signaling mechanism exists that differentially targets GluR2 transcripts with alternative 3′UTRs.